Dreaming Large — Dean Wesley Smith

The Dreaming Large, a Seeders mother ship, vanishes without a trace while approaching the edge of a galaxy.
Chairman West and his crew on the Rescue One must find out what happened to the huge mother ship. And the over half-million lives on it.
But as they approach the galaxy's edge, the last known position of the big moon-sized mother ship, what they find surprises them all.
They find nothing. Absolutely nothing.
A galaxy-spanning science fiction story of the unknowns in space.