We Don't Know ... What the Universe is Made of

What’s the universe made of? That’s the overarching question to everything I do. We know from astrophysical observations that it’s mostly made up of dark matter and dark energy, but we don’t really understand what those are.

Dark matter seems to be like some sort of fundamental particle — something that interacts weakly, if at all, with ordinary matter, but still has mass. If you look at the gravitational attraction of big astrophysical objects — galaxies like the Milky Way — they can’t hang together and behave the way they do without having significantly more mass than we can see associated with them. You can’t explain the structures you see in the universe without some dark matter there.

Dark energy, on the other hand, is even more slippery. It could just be an artifact of general relativity, or it could be something new — nobody knows. It was originally postulated because astrophysicists discovered that the universe is not just expanding, but the rate of that expansion is actually accelerating, so there must be some source of energy blowing it apart. By definition, that’s dark energy — but we have no idea what it means.

Now is a great time to be a physicist, because we know with certainty that we don’t know what we’re doing. That means there’s no shortage of opportunities for discovery. What will probably happen in the future is what has historically happened in science: We’ll find something weird nobody predicted.

My hope is that there will be a weird anomaly that shows up in the data we collect at the Large Hadron Collider, [a massive physics instrument that straddles the French-Swiss border], and we’ll say, “What on earth is that?” Then theorists will look at it, and we’ll get a little more direction.

To put it in very human terms, though, I’m 60 now. If I die and don’t know what dark matter is, I’ll be really pissed off. It really bugs me that we don’t know what it is.

— as told to David Levin