> The simplest explanation is that there is regular matter which we fail to detect. Not weird, given the huge distances involved - there almost assuredly is mass that we didn't detect (e.g. planets).
What is the difference between that and dark matter?
> there almost assuredly is mass that we didn't detect (e.g. planets).
Don’t planets give off light and thus contribute to the distribution of light observed?
The GP is talking about MACHOs; MAssive Compact Halo Objects. They used to be a valid DM candidate but have been ruled out 20-30 years ago. Planets don't give off any non-negligible amount of light. However, they do consist of baryons (read: atoms) which would contribute to the baryonic accoustic oscillations in the CMB power spectrum, so that's a good probe to set upper limits on baryonic matter in general.
Primordial black holes are a somewhat related candidate; but those are mostly considered ruled out as well. They are not considered baryonic matter.
What is the difference between that and dark matter?
> there almost assuredly is mass that we didn't detect (e.g. planets).
Don’t planets give off light and thus contribute to the distribution of light observed?