We observe patterns of behavior – orbits, movement, gravitational lensing – that are exactly what we would see if, for example, there were great clouds of matter or other galaxies in those places.

Which would still not rule out anything else...

But we don’t see the hydrogen gas. We see non-uniform distributions of dark matter mass that imply there is not simply some consistent calculation error, but rather that there is dark matter that is not uniformly distributed.

That non-uniformity though, yes, this is a good point for a "dark matter exists" hypothesis. Although I would still word it differently: Not "We see non-uniform distributions of dark matter mass" but "We see a non-uniform mass-like effect". I've learned that keeping the terms as neutral as possible, or it might exert too much pressure on the thought process to go in just one direction.

We’ve also discovered things like ultradiffiuse galaxies – likely remnants from ancient collisions – that have apparently been stripped of their dark matter.

Which is basically an extreme case on "not uniformly distributed".

MOND cannot explain these observations because these galaxies essentially behave in a Newtonian manner that would be impossible in a MOND framework.

That is acceptable. I was not "selling" MOND here (or any other theory), btw, I'm just wondering what kind of possibilities are there to explain all those observations. "An invisible mass nobody has observed except for it's gravity effect" sounded a bit thin of a leg to stand on there, while incomplete models are a rather widespread phenomenon.

electromagnetism is the reason things clump. Absent electromagnetism, what would cause clumping?

Gravity? I mean, we are talking about something that has gravity. Did planets form because of electromagnetism?

Yep, and more than a handful Many that make specific predictions we can test for and so are testing for.

Indeed. Try that with the wannabe-sciences like economics...

For example, you could look at axions, which are a theoretical particle predicted by an entirely different theory that may be a good fit for the dark matter particle.

Well, at least they share the common trait of not being found yet... ;-)