Sort of.

We have a lot of data that requires something like dark matter to exist. There’s a bunch of matter, and it moves in a way that requires way more matter than what we can see. We can detect gravitational effects pretty much everywhere that greatly exceed what we’d expect from the matter we detect. We can detect this so well that we have intergalactic maps of where this extra matter must be.

We just… don’t know what it is. We’ve basically ruled out all the stuff that we know about. And problematically, everything at small scales like what we deal with in the solar system seems within what we’d expect with normal matter. But when we look farther out, stuff acts like there’s way more gravity than there otherwise should be.

Everything is unknown to an extent. Dark matter however has been known to humans for decades (nearly century) but there are unknowns to it (:

No, not controversial at all. Dark matter is one prevailing theory that explains a number of odd phenomenon and observations that standard cosmology gets wrong. Things like the spin rate of galaxies is an example. There may be other explanations that fix this problem (say perhaps super gravity) but none have been any more confirmed that the others.

Dark matter is also unknown in the sense that other than prescription of what it should be, we don't what it actually is. None of our current particles fit the bill and the theoretical particles we've thought could work haven't shown themselves in any tests to make them.

Dark matter is basically the name specifically for the stuff that is having gravitational effects on things but that they can’t attribute to a proper source. It happens A LOT so they just call it dark matter.

It’s not controversial by any means, it’s specifically an unknown

Nope. We aren't even sure it's there, we're just pretty sure.