I think dark matter is an awful explanation for an explanation of the "missing matter" problem. We can't see it? Oh it must be invisible?!? Lame.
I have another idea and it is much more simple. It starts with the basic question: What if the gravitational effects come from beyond the edge of our universe?
A couple of points of evidence and a concept appropriated from astronomy:
For those that don't know, the heliopause is a sort of sheath or cloud that surrounds our solar system. It is created by interactions between matter, the solar wind, the interstellar wind, etc. You can read about it here.
It is roughly comet shaped because the sun traveling through the universe is compressing this cloud in the direction of travel and stretching it out going the other way.
Now, if you imagine this on the scale of our entire universe in the same way, all the matter and stars and galaxies and such are surrounded by a cloud (what we call the edge of our universe) and whisking through space. To be clear I know all our galaxies have their own unique vector but I think they the entire universe is moving on its own vector and is superimposed much like our sun's vector is superimposed on the Milky Way's vector. Let's call these Local Universes and our universe is one of them.
So how does this solve point 1 and 2 above? Well if you imagine all the matter in our universe is not contained in a single Local Universe but in actuality our universe is really much, much larger and FULL of Local Universes. Each with massive amounts of matter (but none of this dark matter stuff) and each exerting a net gravitational effect on other Local Universes.
The result is that all this tugging would pull or stretch apart the matter (e.g. galaxies, etc.) inside a Local Universe and because there is no reason to assume they would be distributed through space equally it accounts for the uneven expansion of our universe. The number of Local Universes in the Global Universe can be calculated by assuming that the "missing" matter comes from these Local Universes.
This theory is not well-baked for the following reasons:
I would love for someone to model this and at least establish its potential.