Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.

Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There's no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.

For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe. Ever since the first indications of "dark matter" scientists have been attempting to explain it as something more familiar to us, some kind of atomic matter or some-such, maybe gas or dust or lots of planets or dark stars or something. At every single turn they've been stymied, and instead of eliminating the idea of dark matter as an ethereal particle they've instead eliminated other possibilities.

Don't look at the current theory of dark matter (weakly interacting massive particles) as some hare-brained scheme that scientists thought up, instead look at it as the hard-fought victor of numerous observational challenges. Dark matter is the theory that survived. We tried explaining things a zillion other ways (gas clouds, compact objects, neutrinos) and those theories just didn't match the observations. There are also a few exceptional circumstances (such as the bullet cluster) that indicate very strongly that dark matter is something different than either gas clouds or stuff like stars and planets, because in the bullet cluster we can observe the gas and the stars and planets and the mass, and each of them are in different places because each of them follow different rules when it comes to interacting during a galactic cluster collision.

Dark matter was "invented" because there wasn't enough observable mass in galactic-scale objects to account for their behavior. In other words, they acted like they had more mass than we could observe. Dark matter is basically characterized by not responding on the electromagnetic spectrum, which is what we use to do these observations. Since all the matter we know of generally does respond on this spectrum, that's why dark matter is considered to be "exotic".

https://en.wikipedia.org/wiki/Bullet_Cluster is fairly good evidence that dark matter isn't just unobserved regular matter. In these massive cluster wide collisions the dark matter seems to have "kept going" (you see very strong gravitational lensing where there appears to be nothing) while regular matter that we know is subject to forces besides gravity collided together, slowed down, and became very hot.

"Dark Matter" is an unfortunate name, since it sounds like regular matter that is not adequately lit.

In reality, it's something we have no idea what it is, except that it's not visible and a big source of gravity.