What if there's no dark matter?

[Riemann]

@Senegoid

A lot of physics at the cosmological level or the quantum level consist of equations that seem to accurately describe some observed phenomenon, even if the can’t make heads or tails of what “is actually there”, and we can only describe some observed behavior. It’s common to “explain” this by simply giving it a name. Oh, we’ll call it “dark matter”. We don’t really know what that could possibly be, but it seems like a fitting description for some phenomenon we observed and described.

I’m not sure what you mean by what is “actually there”? All we ever have is the observed phenomena. Perhaps I misunderstand, but when you say “they can’t make head of tails of what is actually there”, that sounds to me like you’re saying that the observed phenomena are not consistent with any of our intuitive mental models that are based on the macroscopic classical world. Notoriously, a photon is not “actually” a wave or a particle, although it can sometimes behave like either, and it can (loosely) be in different places at the same time. Under our current model, these observed properties, along with some others, fully define what a photon is. There may arise a more complete model, but it will certainly entail similar weirdness. The fact that this is startling and perhaps unsatisfying is not a failure of physics to explain what is “actually there”, it just means that our macroscopic classical intuition fails in the QM realm.

I don’t see any conceptual difference with the dark matter hypothesis. “Dark matter” is not an arbitrary name. The best hypothesis that we have to explain the data is something which know must have the properties of interacting via gravity (i.e. “matter”) and not by EM (i.e. “dark”). So “dark matter” states precisely the two properties that we do know so far. If the hypothesis is correct, we will eventually flesh out many more properties. And the sum of those properties will be all that it actually is. (I’m not excluding the possibility that the hypothesis might turn out to be completely wrong, although it seems unlikely.)

[Pasta]

@Riemann

So “dark matter” states precisely the two properties that we do know so far.

We actually know many more properties that that, and they are all consist with the dark matter picture. We also have quantitative constraints on…

• how much of the dark matter can be baryonic in nature;
• the average energy density of dark matter in the current universe, related to
• the viable masses of dark matter particles, and
• their number density;
• the required spatial distribution of dark matter near gravitational wells (galaxies, clusters, and larger);
• interaction rates of dark matter particles on standard model particles;
• their velocities at various stages of cosmological evolution;
• their roles in structure formation at various stages of cosmological evolution;
• dark matter self-annihilation and decay rates

and much more. Some of these constraints are intertwined with one another, and some are one-sided constraints. But all told, the type of objects we’re dealing with have a lot more data-driven “character development” behind them than is often conveyed in passing.