First of all, anti-matter and dark matter are totally separate things, so don’t get confused between the two.

Anti-matter particles were formed right after the big bang along with normal matter particles, and they are just the opposite of normal matter particles. Photons converted into matter-antimatter pairs, which annihilated again to form photons in the form of light energy. And yes, antimatter is a real thing.

Dark matter is a type of mysterious, unknown matter that, unlike ordinary matter, doesn’t interact with light in any way. So, we have to detect it based on its gravitational influence that glues many distant objects together.

If you want to find more about dark matter, watch my YouTube video regarding it:
https://youtu.be/WTsNOdYVFMY