As a general category of theories that's called "cold dark matter" — it needs to interact with something in order to clump up / cool off, but perhaps not in a way that's very visible to us. A wikipedia excursion will net you a handful of such theories and their pros and cons.

That's essentially the leading theory. They're called WIMPs.

If this new primordial black hole theory works out, then doesn't that mean that MACHOs beat WIMPs once again?

These theories aren't necessarily at odds with one another. Could very well be a combination of both MACHOs & WIMPs among other things...

Thanks for the search term, I'll look into it.

I would guess that Axions are currently the leading candidate.

Why would the clump if they don't interact?

Gravity still attracts them. They do have mass

But without interacting they have no means to shed the energy they gain when attracted to other matter, so they won't clump like normal matter.

They interact with normal matter and themselves via the force of gravity...

But gravity is (effectively) a conservative force.

For the most part a dark matter particle attracted to another particle will just convert all the kinetic energy it gained during the attraction back to potential energy as it whizzes past and away from the other particle.

Normal matter particles can radiate away photons when they hit each other, ie friction, leading to a non-conservative interaction. This allows them to shed their energy and clump in a way dark matter can't.

This is why there is still a lot of atomic hydrogen in interstellar space. Thermodynamics suggests the atoms should bind together and form molecular hydrogen, but since a hydrogen molecule has no dipole moment, two hydrogen atoms alone in space cannot easily radiate away the energy released when they would bind. So they don't bind.

I used to think this as well. However, all orbiting mass can shed energy via gravity waves, and by interaction that fling some mass out at high kinetic velocity while other mass sinks deeper, which is a primary method of star formation afaik. Basically, radiating each other instead of photons.

There is also the possibility of dark matter only forces and fields that don't affect normal matter, but allow dark matter to self interact to some extent.

No, gravity is different to all other forces. First it's not really a force, then it's attracting and distant. We don't even have a proper theory for it yet.

Sure gravity is different from the other forces. That changes nothing about what I said, because we do have a proper theory for it: General Relativity. General Relativity matches a vast number observations with stunning precision.

Of course General Relativity is not the final answer, we know that. But that does not mean we know nothing about gravity, or that we somehow can't use our current theory to make predictions.

Dark matter as a concept assumes General Relativity is good enough, and takes it as-is. Without modifying gravity we need some other way to explain the discrepancies like galactic rotation curves, and so for dark matter we add in some new, so-far unseen, particles. We also know they have to interact very weakly with normal matter besides gravity, otherwise we'd would already have noticed them in existing observations and experiments. Based on this we can make predictions.

What would happen if we have some matter that only interacts gravitationally buzzing around?

Well as mentioned in my previous post such matter can't cool down and clump like normal matter, so instead it would be spread out in diffuse blobs around galaxies. And when we model galaxies with such dark matter halos we find that indeed, that might explain galaxy rotation curves. It also makes other predictions which also seem to match other observations. So overall dark matter looks like a pretty good candidate.

The alternative of course is to assume there aren't dark matter particles, but rather that GR has to be replaced or modified in some non-trivial way. There are many people working on that.

However they're so far struggling, because it turns out to be very difficult to change or replace GR without failing to match existing observations. So that seems to indicate that GR is a pretty darn good theory of gravity.

I think that meant something like don't interact with our "ordinary matter" in a way that we know.

What if dark matter is actually a secondary effect (a-la-turbulence) of the flow of space that gravity causes? (I know the idea that space can flow is a fringe idea, but it seems thinking broadly/laterally in this area is not a terrible idea?)

That would be a modified theory of gravity.

I don't know about the idea of turbulence but modified gravity is serious science.

It is just not the preferred explanation right now. An important reason is that AFAIK, none of these theories are currently able to match the observations without the dark matter they intend to eliminate.

Especially after we spotted the bullet galaxy.

Two galaxies collide. The dust stops, the dark matter keeps going... it isn't looking good for MOND.

Attempts to modify gravity have not been too fruitful so far.

I've had a theory for years now that it is the opposite: eg that gravity is a second tier effect at shorter distance of normal matter consolidating in order to seek breaking through black holes into dark matter. So some form of entanglement as two universe membranes collide (aka the big bang) means "real matter" in this universe was but is no longer dark matter (this would only work in a dual universe system instead of a multiverse system I posit) but due to all matter entering via a single membrane collision point it is all some form of entangled (quantum or otherwise), so in this theory black holes aren't the dark matter, but the gateway for normal matter to become dark matter, transitioning back to the other universe. In this theory, dark matter would then be concentrated in the areas of our universe with sparse amounts of normal matter, a theoretically testable hypothesis at some point in the future.

I'm no physicist, theory was developed while doing Einstein inspired imagination visualization/simulation of the big bang. I know it's probably so wrong in so many ways it's not worth a normal physicist to respond to, just thought I'd share.

If I were correct, which I'm probably not, it would mean there are no "primordial black holes" that existed since the moment of the big bang, but rather as time went on the black holes were created and increase in number (also being part of what causes this universe to expand)

This is basically equivalent to any other crackpot theory. You can’t through together a bunch of impressive sounding words and expect to get anything out of it. The one prediction I noticed that you made that “ dark matter would then be concentrated in the areas of our universe with sparse amounts of normal matter“ has already been proven to not be true. Dark matter is concentrated in galaxies.

Yep, I get shit on anytime I mention it, despite the disclaimers. Whatever, it's just a fucking thought experiment.

It's not fair to write that you have a "theory", then suddenly convert it to a "thought experiment" when someone points out the the one testable prediction you propose is already disproven. This is equivalent, though obviously lower stakes, to making a racist comment then saying it's "just a joke" when it's received badly.

If you want things to be received as a thought experiment, call it that up front. You'll get people thinking and talking about it. But the response you received is in line with what the word "theory" means in the scientific community.

> I'm no physicist, theory was developed while doing Einstein inspired imagination visualization/simulation of the big bang. I know it's probably so wrong in so many ways it's not worth a normal physicist to respond to, just thought I'd share.

Not sure how much more clear it can be... If the single word "theory" misapplied can't be seen through by such intelligences well what else is there to say than I'm sorry and I'll never mention it again. I'll stick to the theories I'm good at.

When you use word with mostly clear Technical definitions and meanings, you open yourself up to refutation. What you wrote is almost equivalent to saying 2+2=47374636628