Of course they're just placeholders for things we don't understand, but my guess is that they are not any form of energy or matter at all, but a misunderstanding of the geometry of space or something similar.
Luminiferous aether was invoked to explain the ability of the apparently wave-based light to propagate through empty space. [1] Eventually we found evidence that contradicted that idea.
Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. [2] We are searching for direct evidence, but haven’t found any yet.
They will call that "baryonic matter".
I'll see myself out.
Don't astronomers call elements like neon metals? Maybe cosmologists don't?
What other people have been doing over the past 40 years is attempting to devise tests for these various dark matter candidates. We know, for example, through lensing observations, that MACHOs/brown dwarfs don't exist in the required numbers and that the neutrino mass seems too low. The problem, of course, is that there are only so many ways to try to observe matter that is truly dark.
I agree, though, that, in the end, it may be that dark matter will be an untestable hypothesis, just like quantum gravity or whatever.