[camacazio]

There's a theory of one alternate "shadow universe" that accounts for gravity holding stars in place in this galaxy when the mass of the seen galaxy does not account for the gravity holding the stars. Gravity can pass through all dimensions, from our percieved 3 to the discovered 11. Light on the other hand cannot cross dimensions in such a way. That's how gravity can affect this universe from the shadow world, but we cannot see it because it's light doesn't cross the dimensions.

As for big bang, even after parts of theory of Special Relativity (opposed to General Relativity) have been disproved, the part about Big bang holds true to all present theory, including the passage of time. What happened before the big bang cannot ever be realized, and there's absolutely no evidence to say that it'll happen again or anything like it happened before. Just that it is physically proven that the universe is expanding and there must have been tremendous energy to start the push.

I can't really explain it all, but there's a nice easy to read and very colorful 100 page book it's in. Read "The Universe in a Nutshell" by Stephen Hawking.