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The problem is if you disregard dark matter, you're then required to disregard the notion of dark energy...
The issue then, is that you have the universe, which expands at a relatively steady rate, however this rate indicates that there is more matter in the universe than we can understand or observe.
Under Friedmann's model of the universe, there are three possible models you can extrapolate..
1. the universe is expanding sufficiently slowly that the gravitational attraction between between the different galaxies causes the expansion to slow down and eventually stop. The gravitational attraction will then cause the universe to begin to contract.
2. the universe is expanding at such a rate that it will never become static, although gravitational attraction between dense bodies will cause the expansion to slow a little..
3. The universe is expanding at a rate only just fast enough to avoid collapse, the speed at which galaxies are moving apart gets smaller and smaller but never reaches a point universal contraction...
Model 1 would require the galaxy to be significantly more dense with particles, galaxies, etc. There would simply need to be much more matter in the universe for the current rate of expansion to be stopped.
Models 2 and 3 would require significantly less matter than we know to be in the universe...
Alternatively, as Steven mentioned, you can "modify" gravity in order to compensate for the lack of matter. After all gravity is one of the weakest forces in the universe.. However this introduces different problems, As steven said.
There needs to be something else out there that exerts gravitational force in order for the universe to stay together at its current rate. Back in the old days Einstein had the idea of a substance called "ether" that was present everywhere, even in the cold vacuum of space... Now, this was used in the early days of relativity... Perhaps he was on the right track, although he never considered the fact that Ether could have a mass, and there for, an element of gravitational attraction...
Great topic when you're looking for food for thought.
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