While there may be a part of it being "different gravity", dark matter cannot 100% be explained by modified gravity of any kind.

Why do we know this? Well there are observable galaxies that survived collisions and have been stripped of their dark matter, and the reverse is also true (galaxy-sized dark matter blobs without baryonic matter in it).

I can refer you to this wonderful PBS Spacetime video about it: https://youtu.be/5t0jaE--l0Y

We know that's not the case because we can see different galaxies with different levels of dark matter.

Dark matter doesn't interact with anything else except by gravity, we don't know why, but we can detect that behavior by seeing the way it clumps together.

We can also see that galaxies that collide with each other have different levels of dark matter than galaxies that haven't recently done so. The dark matter appears to just pass through each other and continue on while the regular matter hits each other and stays generally together in one group.

It's pretty interesting when you work through the details of what we do and don't know.

I'm not entirely 100% dark matter exists in galaxies the way often described. ... The way I see it, it might as well be a repulsive force between galaxies opposed to the current understanding of it being am attractive force. Plus, if it were a phenomenon that pushed things apart, it could also explain Dark Energy.

And to me, that's a perfectly valid theory. Like other proposed explanations for dark matter or dark energy or "whatever the hell it is we can detect the effect of but can't identify", it's difficult to test.

That's why I enjoy science. It's like a big puzzle, and sometimes you get halfway done and realize you put it together wrong and have to start over.

Fair enough. I believe it's ducks. Ducks with ambition.

Plausible!

I had a bachelor's in physics a decade ago.

But here's how my memory describes how we discovered, or at least how we did it in my computational physics class.

You have stars of known size, and there for light output as its directly proportional to size. You also have a known distance.

You can then calculate how bright the star should be. But its wrong.

Meaning there's things in the way thats blocking light.

So we call it dark matter because it hasn't been directly observed and its clearly there. It could be our fundamentals are wrong, but that's unlikely.

It could very well follow gravitational fields, and then attracted to galaxies with large masses.

But it could also be something in the vacuum. We just have no evidence to suggest either way.

Wouldn't that mean that that force would be stronger on the edges of the galaxies, instead of the center? I imagine this is something we could figure out

I guess on a similar note, my own wacky theory is that our dimension can be affected at any given time by up to 13 other dimensions, but which 13 can change amongst a potentially infinite number. I imagine certain dimensions would more likely be co-terminus (I term I believe I'm borrowing from a Dungeons and Dragons type source) with ours than others but who knows.

The primary thing we have detected is an attractive force within galaxies. Whether that's otherwise-undetectable particles or a mistake in how we calculate gravity or something else, we definitely know it's that there is more attractive force holding galaxies together than there should be based on detectable matter and general relativity.

Simply put: galaxies rotate too fast. Much, much too fast. That can't be caused by repulsion between galaxies. Only by the stars within a galaxy being pulled towards the centre of that galaxy my than we would expect. Similar to how you have to spin faster to hold a big bucket of water horizontal without spilling than to hold a small bucket of water.