science

isn’t antimatter like a positron just oppositely charged form of all particles (or opposite in all forces I guess)

They annihilate because they have anti-mass

Antimatter has opposite charge, same forces, same mass. They experience and exert all the same forces in all the same ways, including gravity.

antimatter and matter colliding could result in these massive particles that can no longer possibly interact with anything. Isn’t that what Dark Matter is? Seems we have a lot of that stuff floating around.

No, during annihilation, especially in low-energy collisions, you usually just get two gamma ray photons out. At higher-energy collisions, you do get massive particles (as in particles that have mass, not big huge particles), but they're perfectly ordinary (well, relatively ordinary, anyway). Wikipedia has a pretty good overview: https://en.wikipedia.org/wiki/Annihilation

I saw another unhinged amateur theory a few weeks ago also on reddit that Dark Matter was the next universe moving backward through time.

No, dark matter is also relatively ordinary matter as far as we know, except for the fact that it doesn't really interact with other matter or energy except gravity. Technically it's also hypothetical, because there isn't any nearby for us to get a good look at or take a sample, but it's our working theory. Because it appears to interact normally with gravity (i.e. dark matter gravitates toward other matter and bends light as expected) it's almost certainly not a mirror universe doing something funky. Plus, the dark/normal matter ratio in the observable universe is 85/15%, so if it was a mirror universe, wouldn't it be 50/50?