What really are dark matter and dark energy?

[Tanya Li]

Given that Dark Matter and Dark Energy are estimated to account for roughly 95% of the universe's total mass-energy content, the fundamental nature of these phenomena remains a critical question in modern cosmology.

Currently, the standard Lambda-CDM model treats them as distinct, physical entities, such as weakly interacting massive particles (WIMPs) or a cosmological constant. However, alternative hypothesis suggest that these observations might instead indicate a breakdown in our current understanding of General Relativity at galactic scales, pointing toward theories like Modified Newtonian Dynamics (MOND).

As a student analyzing these different frameworks, I would like to ask which direction the scientific community views as more promising. Is the consensus still firmly in favor of discovering actual particles in the "dark sector," or are modified gravity models gaining serious mathematical traction?

 

[Dale]

Is the consensus

There isn’t a consensus at this time. The community is pursuing multiple avenues, both theoretical and experimental. Individual scientists have their preferences, but there is no consensus yet.

 

[Ibix]

I don't think there's any consensus.

We're fairly sure GR isn't quite right, so hints of alternate gravity theories are attractive, and MOND is embarrassingly good at modelling galactic dynamics. But because it's Newtonian at "normal" field strengths it is worse than GR at solar system dynamics and it doesn't work on cosmological scales at all, and efforts to invent a modified GR that is MOND in the relevant limit haven't panned out. And the MOND regime isn't really where we expect GR to fail anyway. So MOND may be telling us something about gravity, or it may be telling us some as-yet-unknown regularity about galactic dynamics.

Dark matter is the simpler hypothesis in some senses. And there is some limited evidence of it in other contexts - including it in models of the early universe explains the presence of one otherwise-unexplained peak in the CMB fluctuation power spectrum. But direct detection efforts have so far come up empty (except for one recent possible signature in the LUX ZEPLIN collaboration), and without reliable direct detection it's open to the criticism that it's unfalsifiable - you can just find the difference between our models' predictions and our measurements and plug in dark matter with whatever distribution you need to fill the gap.

So I think both approaches have significant challenges, and whichever you prefer you should be prepared to be wrong.

 

[Matterwave]

As other posters have said, there is no consensus on either DM or DE. We don't know exactly what they are.

However, for the dark matter, it is generally thought more plausible for it to be some kind of matter (rather than MOND) due to evidence like the mass distribution calculated from gravitational lensing vs from aggregating radiative sources in the Bullet Cluster: https://en.wikipedia.org/wiki/Bullet_Cluster and other similar clusters.

MOND was originally built for galactic rotation curves, but since then, we have found more evidence in other sectors that present difficulties for the theory.

Still, the issue probably won't be settled until we actually detect (the) dark matter particle(s) and understand its/their properties.

There is a reason the Lambda-CDM is the current preferred (at least baseline) cosmological model. But one must admit that there are some real tensions (e.g. the Hubble tension) in this model, especially as of late.