It’s an amazing fact: Only around 5 percent of the universe consists of the matter that we can see. Physicists noticed some decades ago that the stars on the outer edges of galaxies were orbiting around the center of those galaxies faster than predicted.
To explain this, the scientists suggested that there might be some unseen “dark” matter in those galaxies that caused the stars to rotate more quickly. After this, observations of the expanding universe led physicists to conclude that there must be a lot more dark matter out there—five times as much as the matter we can see.
Alongside this, we know that the expansion of the universe is actually accelerating. This is strange because we’d expect the gravitational pull of matter—both “light” and “dark”—to slow down the expansion of the universe.
Combine this with the fact that the universe is flat—space-time, overall, is not curved—and cosmologists need an explanation for something that balances the gravitational attraction of matter.
“Dark energy” is the solution. Most of the energy in the universe can’t be locked up in matter, but instead, it’s driving the expansion of the universe. Physicists believe that at least 70 percent of the universe’s energy is in the form of dark energy.[3]
Yet to this day, the particles that make up dark matter and the field that makes up dark energy have not been directly observed in the lab. Observing dark matter is difficult because it doesn’t interact with light, which is how observations are usually made.
But physicists are hopeful that dark matter particles might be produced in the Large Hadron Collider (LHC), where they could be studied. It could turn out that dark matter particles are heavier than anything the LHC can produce, in which case it might remain a mystery for a much longer time.
Dark energy is supported by many different observations of the universe, but it’s still deeply mysterious. In a very real sense, it may be that “space just likes to expand” and we can only see it expanding when we look at very large scales.
Or maybe the dark matter and dark energy explanations are incorrect, and an entirely new theory is needed. But it would have to explain everything we see better than the current theory before physicists will adopt it. Even so, it’s incredible to think that we may know very little about 95 percent of the universe.