Let me start by saying that I am neither an astronomer nor a physicist, so what I'm about to suggest may be very obviously wrong, but... Often, the universe is described as a bubble that began with the Big Bang and has been expanding ever since. What if, instead, the universe is actually a large collection of bubbles (like you might see in a soap dish)? Could it have been that the universe not only began with a single Big Bang, but was followed up with multiple smaller bangs that created pockets or bubbles connected to the original "bubble"? My thought is that, perhaps, this model could explain dark matter in that the "missing" matter is not missing after all, but rather is just hidden in the side pockets. If you imagine two overlapping circles and you stood anywhere inside those circles, you would only be able to see those things in a direct line that did not go outside of the circles. Therefore, there may be items that may not be visible to you, but those items might have measurable effects on what you could see. Scaling this idea up is what lead me to think of the multi-bubble universe. Does this make sense?