Wow, that’s a really stupid crackpot theory you’ve found. Almost as bad as the angular momentum guy

Because MOND doesn’t really explain the data. For one, it doesn’t really explain all the mass but for another, the bullet cluster is kinda a death sentence for MOND. Like good scientists, we can’t rule it out completely yet but dark matter explains all the discrepancies much better than MOND and so MOND is a fringe theory among physicists. In addition to the Bullet Cluster, MOND also poorly fits several other features which dark matter explains naturally, including much of cosmology. Less damningly, MOND requires a rather complex, random looking change to physics while dark matter is actually quite simple. We are pretty convinced there it is possible there are particles we haven’t seen yet so it’s not terribly surprising some of these could have astronomical effects. Also, I suspect you have a hard time grasping much of physics (QFT and GR for example) but QFT has been confirmed at the highest precision of any scientific theory ever. The fact laymen struggle with some physics isn’t a good reason to believe something else.

This is a good explanation but there’s also just the simpler fact that if things can’t collide (and dark matter is mostly collisionless as famously seen in the bullet cluster) it’s hard to get it to “stick together”. So while gravitational force will get it form “dense halos” it’s fairly unlikely you’ll get something like planets or stars.

You’re missing a lot of the easy physics to this. Photons can (mostly-or maybe entirely) pass through dark matter for the same reason neutrinos can pass through earth. Dark matter must not couple strongly to the electromagnetic force. This is not terribly surprising. It just means dark matter must be (electrically) neutral. It also can’t couple strongly to the weak or strong force (otherwise it would be easy to observe). This again isn’t terribly shocking (a low coupling is easy to put in). Therefore, we know dark matter only interacts strongly through gravitation. This will mean it will probably never form a structure like a star but even if the densities got that high, it wouldn’t result in fusion since there no reason to believe dark matter is capable of forming hydrogen/helium analogs. There are some people who talk about a “dark QED sector” which would have dark photons and other things but so far nothing we’ve tested for dark matter has panned out and this is where the interesting physics lies.

MACHOs (massive objects like black holes) have been pretty conclusively ruled out by lensing studies. Which leaves us with particles (WIMPs and axions being the most discussed).

This simply isn’t true. Your chances are bad but not because you are slightly older but rather because it is unlikely for any given person to get a research job. If you are passionate and do well in your studies, you’ll get into a great PhD program and then you’re odds are still against getting a research position.

What you can do is try to increase your odds as much as possible. What that means at this point for OP is trying to get into a good PhD program. This will be best accomplished through decent grades and some research experience. Keep going and you have as much a chance as anyone. But understand that these are competitive jobs that usually require 4 years of college+5 for PhD+a few postdoctoral positions+a bunch of good luck-all competing against people who are passionate about the topics and generally intelligent.

The second is closer to the modern scientific definition. And the part that’s problematic is “tested by further observation”. You’ve given a pretty vague analogy that can’t be (reasonably) tested because it isn’t really a full fledged idea. I can ask something like “what if dark matter is made of the souls of the damned” but unless I present a clear idea of what that means (clear enough to obtain potential effects) it’s not really science in the modern sense.

If you’ve spent years absorbing this information and this is the limit of what you know, you haven’t been very efficient. This is neither a theory nor a hypothesis (at least, not in the modern scientific sense). It’s “not even wrong”.

I know it’s frustrating to not understand something you wish to understand, even after putting in some cursory effort (usually, watching some YouTube videos or pop science documentary) but you have to remember that people spend years of their life intensely studying this stuff and learn actual material in that time-not just vague analogies. But if you want to have interesting ideas, you have to actually understand the underlying material. For example, can you derive why a specific rotation curve implies a specific density distribution? That’s basic undergraduate physics (and obviously important for dark matter studies).