[Kuroko]

Matter in the universe is accelerating in it's expansion, at this point, scientists are blaming dark matter for the countering of gravity's effects.

Erm I believe newscientist.com had some thing headlined "Dark matter fighting gravity and winning". Which was an artical on matter accelerating away from each other as though being pulled.

Now, as for your theory on time dilation.
This is in direct line with my theory, so I'm more then glad to accept it as a reasonable explination of effects denser space may cause.

Neither of them have truelly lost time, here's why.

The time taken for both travellers to reach point B is undoubtably different, traveller A however has passed through more space thus taking more time.

But when one tallies up who travelled the furthest traveller A would have travelled obviously more then traveller B.

Your elastic band analogy works fine for this line of reasoning as well.

For traveller B however, there comes a tricky obstacle, does traveller B avoid the denser space, or does traveller B have a means to be unnaffected by denser space.

This would be the interesting question as to travel arround the denser space would be to travel in an order least efficiant of all (which is to say the shortest distance between any two object is a straight line) however depending on the density of space we are talking about this may not be the case.