For the laypeople: dark matter is literally defined as "the gap between our existing model and the data we've collected". It is not a model, it is merely an acknowledgment of a lack of one (see: the vagueness of the name "dark matter").

There have been proposed models for why this occurs (WIMPs, etc.) but none have been convincing from a scientific perspective, mainly due to the inherent lack of observations which can justify such models.

It is good to think about, maybe the dark stuff are the cause of these "early" galaxies.