system
Given the following post, respond as if you're commenting on Reddit:
human
Silly question I'm sure, but what exactly is "dark matter" and why is it important?

I greatly enjoy reading about the time, space, and the universe. It's amazing how complicated it is and how little we know of it. However, while I find the information interesting, I don't necessarily understand it because I have no in-depth knowledge of physics and other jargon that always pops up. 

So, I was be much appreciative if someone could kindly explain to me the subject of dark matter in simple-man terms. Thanks!
gpt
Dark matter is *matter* that's *dark.* It's one of those cases where physicists call something precisely what it is.

It's *matter* because it *gravitates.* That part's fairly obvious, really. But we don't call it *dark* because we can't *see* it — although that's true. We call it dark because it can neither emit nor absorb light, because it's *electromagnetically inert.*

Because it's electromagnetically inert, it doesn't *collide.* If race cars were made of dark matter, the races would be very boring because crashes would be impossible. (As would steering, and the cars themselves, but it's just a metaphor.) Dark matter is *transparent* both to ordinary matter and also to *other dark matter.* Which means if you have a particle of dark matter with some amount of momentum, it's just going to keep going in a straight line forever and ever, because it'll never *hit anything* to knock it off its trajectory.

Because dark matter has this property — this *collisionlessness,* this *transparency* — it tends to exist in large "clouds" around galaxies, clouds poetically called *halos.* As galaxies formed, some ten-ish billion years ago, matter collapsed toward points of higher density. The stuff that interacted — non-dark matter, or *baryonic* matter as we call it — collided a lot, shedding kinetic energy and thus falling into lower-energy orbits around these densities in the intergalactic medium. Over a long time, galaxies formed, complete with stars and all that rot, because baryonic matter has a tendency to stick together and do interesting things.

Dark matter, on the other hand, was oblivious to all this. It just kept orbiting the galactic centres the way it always had, sleeting through anything in its path like water through a sieve.

But it still gravitates! And *how,* does it gravitate! Something like three parts in four of all the gravitating matter in the universe is dark matter, and it shows. Dark matter keeps galaxies stable, and there's a good chance that it helped them form in the first place by creating density without interacting.

If you're interested, read up a bit on galactic halos, and also on the Bullet Cluster. Pretty much everything you need to know about dark matter — except, of course, *what it actually is,* a question that we haven't yet answered — can be learned from the Bullet Cluster.