Dark matter, by its very nature, is unseen. We cannot observe it with telescopes, and nor have particle physicists had any luck detecting it via experiments.
So why do I and thousands of my colleagues believe most of the universe’s mass is made up of dark matter, rather than the conventional matter that comprises stars, planets, and all the other visible objects in our skies?
To answer that question you need to appreciate what dark matters can and cannot do, understand where in the universe it lurks, and realize that “dark” is just the start of the puzzle.
Our dark matter story starts with speed and gravity. Throughout the cosmos we see objects traveling in orbits under the influence of gravity. Just as Earth orbits the Sun, the Sun orbits the center of our galaxy.
Unlike our Solar System, whose mass is dominated by the Sun, mass in our galaxy is spread across thousands of light years. As one moves to larger distances from the galactic center, the stars and gas enclosed within this radius increases. Can this additional mass explain the vast speeds of the most distant stars in our galaxy? Not quite.
What’s going on? One possibility is that a vast amount of unseen mass extends beyond the stars and gas. This is dark matter.
Indeed, the work of Zwicky, Rubin and subsequent generations of astronomers indicate there’s more dark matter in the universe than conventional matter.
As dark matters mostly interacts via gravity alone, it has some curious properties. A cloud of hot gas in space can lose energy by emitting light, and thus cool down. A sufficiently massive and cold gas cloud can collapse under its own gravity to form stars.
We can see the influence of dark matter puzzle not just today but in the distant past, right back to the Big Bang. But the fact remains that we are yet to detect dark matter directly.
This doesn’t particularly bother me, as physics has a history of particles that have taken decades to directly detect. If we haven’t detected it 20 years from now I may be concerned, but for now I’m betting that dark matter is the real deal.