Voids are not completely empty, but merely under-dense regions. They were thought to be completely empty at the beginning, but afterwards some galaxies were found inside, specially dwarf blue galaxies. The so-called Local Void, for example, is crossed by wispy filaments with galaxies.
The so-called voids are detected in surveys like SDSS. We are talking about mapping very distant regions Mpc away, so part of its 'emptiness' is simply due to the limiting magnitude of the surveys, but very probably a certain number of fainter dwarf galaxies might be found inside them in the future.
You may want to visit Brent Tully's personal webpage. He is a leading expert in voids. As additional material for his paper 'Our Motion Away from the Local Void' 2008, ApJ, 676, 184, he did some very nice animations: http://www.ifa.hawaii.edu/~tully/
And that is a very good example, since in that paper I think he gave an estimate of the size of the Local Void, based upon the proper motion of a galaxy that is inside.
This post imported from StackExchange Physics at 2014-06-14 13:00 (UCT), posted by SE-user Eduardo Guerras Valera