More likley to be interdimensional travel, than travel through space as teh distances are too vast, although if tears in space existed, this would provide a shortcut to the other side of the galaxy, we are probably too insignificant to matter.
another theory i think a lot about is the fact that civilisations are too inward looking and over agressive, they end up blowing themselves up before they can progress enough to find out the universes secrets, in effect they never get to tech level one so , all civilisations can never travel too far.
Logged