Is it possible the many-worlds hypothesis explains dark matter? To provide context for the title, is it possible that dark matter is just gravity from other 'branches' of the universal wave function that have split from ours, that are weakly interacting with the gravity in our branch? Is this an idea anyone has explored before?
 A: Interesting idea, but ... no. It does not work like that. The branching idea is a bit misleading. Instead, the Many Worlds interpretations refers to the terms in the superposition of the wave function of the universe. Each terms in this superposition is viewed as a "world," which is perhaps a bit misleading. So all possible "worlds" are present in the wave function all the time, but with different coefficients.
This wave function undergoes unitary evolution. What it means is that whenever an interaction takes place the coefficients in the superposition are changed through a unitary process. Such a unitary change maintains the overall normalization of the total wave function. So it is not really the case that the universe branches off into many universes.
How gravity enters the picture is not fully understood yet. For that, one needs a theory of quantum gravity, which is still a work in progress. However, it is reasonable to argue that it would involve the entire wave function and not just the individual terms. So the "worlds" should not give different gravity effects. They should all act together to produce a coherent gravity effect. However, this understanding is still rather speculative.
A: 
Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.

In the google search "many worlds dark matter"   there is this arxiv paper  , and others that show people are researching other dimensions for explanations of dark matter. research is on going.
