[uoaei]

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.