system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Dark Matter

I tried to read a bit about it but it was all a bit to scientific for me. Would like to know why we have it and what role does it play in our cosmic universe like i am 5.
gpt
We have a pretty good idea of how gravity, motion and mass all work. We are pretty good at figuring out how big things are, how much mass they have, and how fast they are moving. When we look at a galaxy, we like to  figure out things like how much mass it has, how fast it's rotating, and how big it is. We even take a close look at how fast the galaxy is rotating at different points, for example the middle, the outside, and places in between. 

From what we know of mass, gravity, and motion, we would expect that far edge of a galaxy would rotate slower than a spot closer to the middle. If you make a graph of the speed stars are rotating about a galaxy versus how far away they are from the center, it's called a rotation curve. That graph should look like a children's slide. It goes up really quickly from the center, and then slides down and down as you go towards the edge of the galaxy.

The problem is, it doesn't. When we actually look at galaxies, and draw the graph, it looks like a cliff instead. It goes up quickly (as predicted), but then it stays more or less the same, all the way out. So now we have a problem. Something is wrong with the picture. It's possible that we don't know as much about gravity as we think. However, every experiment we've tried, even really, really complex ones, tells us that we've got gravity nailed. It's also possible that we don't understand how to measure speed and mass from a long way away. Again, however, we've done a lot of experiments, and they all tell us we know what we're doing. So we need a way to explain why the galaxies aren't moving the way we expect.

There are a number of theories that try to explain this. The dark matter theory basically says that there is a lot more mass than we thought in a galaxy. When we estimate the mass of a galaxy, we're mostly doing it by looking at the things we can SEE, like stars (nice and shiny). But what if there was a lot of mass we COULDN'T see? Well, there are black holes. But the truth is we can still 'see' those by the way they warp other stars light, and make them move. Even accounting for black holes, galaxies are still way short on matter to explain the motion we see.

So what if there was matter we couldn't see, but it was really spread out, so we wouldn't see it bending light, or moving stars around? What if there was a whole bunch of it? Well, it turns out that if there WAS a bunch of matter spread around that we couldn't see, then all our observations would actually work out again.

There are other explanations for why our observations may be off, but most are dependent on us being wring about things we're pretty sure we're not wrong about, like how gravity works. There are also other observations that don't match reality unless we add in 'dark matter'. So right now, assuming that there is a crap-load of matter in the universe that we simply can't see is the best way we have to explain why our observations don't match our predictions.