About 95% of the so-called known universe is a total mystery. We have no clue what it is except that it's weirdly different from any sort of matter or energy humans actually know anything about.

The stuff we do know about makes up a scant 5% of the universe and includes everything and everyone on Earth along with every planet, star and galaxy in the universe. The rest is literally invisible, although scientists have managed to detect its immense influence on the otherwise inexplicable motion and structure of galaxies. Revealing the undiscovered properties of the mystery 95% — collectively comprised of what's referred to as dark matter and dark energy — will require a universe-spanning project far more comprehensive than anything done before.

In 2025, the NSF-DOE Vera C. Rubin Observatory will begin a 10-year mission to do exactly that. Jointly funded by the U.S. National Science Foundation and the U.S. Department of Energy's Office of Science, the observatory is uniquely different from others. From its home atop a remote desert mountaintop in Chile, Rubin Observatory aims to capture the entire visible southern sky over and over again with unparalleled completeness, detail and speed. Every change in the visible sky will be precisely recorded, measured, catalogued and analyzed.

"We're making a digital color motion picture of the universe," says Rubin Observatory Chief Scientist Tony Tyson, who is also a Distinguished Research Professor in the Department of Physics and Astronomy at UC Davis. "It will contain information that we can get in no other way."

Rubin Observatory's sentinel-like mission is expected to yield a staggering number of new discoveries: over 17 billion Milky Way stars, about 20 billion galaxies and around 10 million supernovas (over a thousand per night), plus a slew of comparatively nearby objects in our own solar system, including millions of asteroids and untold numbers of comets and interstellar objects just passing through.

"Imagine looking at a cluster of galaxies and watching the supernovas go off in timelapse. You'll be able to look at that," says Ed Ajhar, NSF program director for Rubin Observatory. "These are capabilities that never existed before." In total, Rubin Observatory will collect more data than all previous telescopes combined, he says.

Besides the inestimable value of this trove of science-grade data, the spectacle of millions of exploding stars should also be pretty cool to watch.

It's dark. It's like matter. That's about all we know.

Every schoolkid learns about atoms, the building blocks of matter comprised of protons, neutrons and electrons described in various configurations on periodic tables hanging on countless classroom walls.

But nowhere on the periodic table will you find dark matter.

Coined by Swiss astronomer Fritz Zwicky in the 1930s, who first discovered evidence of enormous quantities of unknown matter in distant galaxies, the name succinctly describes just about all that is known about dark matter to this day: It doesn't interact with light, yet it exerts gravitational force like anything else with mass. Zwicky's findings were largely ignored for decades.

In the 1970s, astronomers Vera C. Rubin and Kent Ford were studying the motion of stars in spiral galaxies by measuring the spectrum of light emitted by the stars. The apparent spectral change of the starlight, which shifted either bluer or redder, revealed the respective speed of the stars as they orbited the center of their galaxy. Rubin and Ford observed that, contrary to expectations, stars far from their galactic centers were orbiting just as fast as stars closer in. At such speeds, stars on galactic edges should escape the gravitational pull of their galaxy and speed off into space. The gravitational force from something invisible yet massive must be retaining them.

Rubin realized they had found the most conclusive evidence yet for Zwicky's mostly forgotten dark matter. She calculated that dark matter must outnumber visible matter 10:1 in spiral galaxies in order to exert sufficient gravitational influence to keep those speedy outlying stars in their orbits. Rubin's work — and later others — confirmed that dark matter can only be indirectly observed through the gravitational effects caused by its proportionately overwhelming mass.

That's because, other than gravitational attraction, dark matter does not interact with light or any known type of matter — both pass through it seemingly undisturbed. No other physical properties have been discovered.

"Dark matter is outside the standard model of physics," says Tyson. "We have no idea what it is. With Rubin Observatory, we now have a really good chance of looking at its properties," he explains. "And once you understand the properties of something, then you can reverse engineer what it probably is."

In honor of Rubin's pioneering work, Congress officially renamed what had initially been called the Dark Matter Telescope, and later the Large Synoptic Survey Telescope, as the Vera C. Rubin Observatory in 2019. Rubin has been an inspiration to many, not only through her scientific excellence but also through her legendary grit in the face of bias she contended with throughout her career as a female scientist.

She also inspired her four children, who saw their mother's passion for galaxies and stars firsthand. Her youngest, Allan Rubin, remembers his mother and father (a mathematician at the National Institute of Standards and Technology, then the National Bureau of Standards) working on their research at home most evenings and discussing it at dinner with the family.

"We had a small dining room table where we ate and a large dining room table for company, but the large table was mostly spread out with their papers and their work," he recalls. "After dinner, they'd sit at that table working."

Like all the Rubin children, Allan Rubin became a scientist. He studies fault lines as a geophysicist. "I went into science because of my parents and seeing them work around that dining room table," he says.

Because of Vera Rubin's findings and others who followed in her footsteps, scientists have calculated that known matter makes up only about 5% of the universe, while dark matter makes up about 27%.

So what's the remaining 68% made of?

Plot twist: Energy from the void

In the 1920s, astronomer Edwin Hubble observed that stars in distant galaxies appeared to be speeding away from Earth, revealing that entire galaxies are moving away from one another in space and thus the whole universe is expanding. The discovery has allowed researchers to more accurately calculate the age and size of the universe and provided key evidence supporting the Big Bang theory of the universe's origin.

Decades later in the late 1990s, two groups of researchers independently found that the light emitted by certain types of exploding stars in other galaxies was unexpectedly faint. The surprising reason is that, contrary to the thinking at the time, the speed at which all galaxies in the universe are zooming away from each other is not constant: The universe's expansion must be accelerating.

Saul Perlmutter, one of the researchers who made the discovery and later shared the 2011 Nobel Prize in physics for it, called it a "plot twist." To comprehend its fantastic implications, imagine throwing a baseball. But rather than slowing and eventually hitting the ground, the ball suddenly starts accelerating faster and faster until it shoots off into space, where it continues accelerating. The researchers' findings showed this is basically what every galaxy in the universe has been doing for about the past 5 billion years.

The amount of energy required to continuously accelerate all the stuff in the universe — hundreds of billions of galaxies made of known and dark matter alike — is incomprehensible and exponentially greater than any known type of energy. The combined nuclear furnaces of every star in every galaxy are puny by comparison.

Enter "dark energy."

When it comes to understanding the nature of dark energy, it's all about precision measurements, explains Kathy Turner, program manager for Rubin Observatory at the DOE Office of Science. "Rubin will sweep back and forth across the sky for 10 years, and each object it observes will be measured repeatedly. From that, you can unfold the dark energy. We're going to get a world-leading number of different astronomical objects measured and with exquisite precision."

Another mysterious aspect of dark energy is that its potency is apparently undiluted by the volume of space it occupies. For example, the force of gravity weakens as the distance between objects increases. Not so with dark energy, which exerts the same amount of force even though the space between galaxies is rapidly increasing as the universe's expansion accelerates.

"Our supposition is that it is the energy from the void — an energy that is everywhere in the universe," says Agnès Ferté, who studies dark energy as a member of Rubin Observatory's scientific team and a cosmologist at DOE's SLAC National Accelerator Laboratory.

It might not be energy at all but a misunderstanding of how gravity works at a universe-sized scale.

"Is gravity really what we think it is?" asks Ferté. Albert Einstein's theory of general relativity accurately explains observations of gravity's effects at the scale of planets, stars and even entire galaxies, she says.

But does general relatively still hold up at the scale of billions of light-years spanning billions of galaxies? "If not, then maybe the acceleration of the expansion comes from that misunderstanding of gravity," says Ferté. "It would be a huge breakthrough if we show that we need a different or more complete theory of gravity. I can't even imagine what sort of applications we might have for that."

The turning point

As Rubin Observatory carefully measures the far reaches in search of data revealing the substance of the universe, it will also provide an unparalleled catalogue of objects much closer to home: millions of previously unseen asteroids within our own solar system, along with interstellar objects that originated in other systems and traveled to ours.

"This is how we're going to understand the origins of life," predicts Ajhar.

The conditions under which the Earth formed and eventually developed an environment that allowed life to evolve are mostly unknown. Large numbers of rocky and icy asteroids from those early days are still present in our solar system, but we have not previously had the ability to observe more than a few. The millions of asteroids that Rubin Observatory will spot are expected to provide key evidence about the early history of Earth. They may also be useful in identifying the factors that other solar systems would likely need to have Earth-like planets.

"Knowing how common such solar systems are and all the details on how it works for life to evolve long enough to give rise to people — we don't yet know any of those things," explains Ajhar. "You don't know what you don't know. For example, is a big planet like Jupiter needed to draw asteroids away?

"You need something on the scale of Rubin Observatory just to figure out where all the asteroids are, and we can only see one solar system well enough to do that: ours."

Rubin Observatory's mission — the aptly named Legacy Survey of Space and Time — will last until at least 2035. The high school students of today will be the early-career astronomers and physicists of tomorrow, examining and exploring all that rich data.

"I think Vera would want those students to understand that most of what there is to know about the universe is still not known," says Allan Rubin. "Vera believed there is always more to be discovered."

While Rubin, Zwicky, Perlmutter and many other researchers have been exceptionally successful in discovering mysteries that show just how little we understand about the universe, Tyson is optimistic that we are approaching a transition in human understanding of the cosmos, from uncovering more mysteries to actually solving them.

"It is a turning point because we're going to be able to do something totally new with very high precision," says Tyson.

"I think we're going to discover something that blows our minds."

This article was written by Jason Stoughton. It originally appeared on the U.S. National Science Foundation's website.