Complex sentence: To sate their hunger, the Wraith tried to get to Earth, whose population is much bigger than that of the whole Pegasus Galaxy; this can only be achieved either through the Stargate or getting more advanced hyperdrives, both present in Atlantis.
Split into simpler sentences: To sate their hunger, the Wraith try to get to Earth whose population is much bigger than that of the whole Pegasus Galaxy. This can only be achieved either through the Stargate or by getting more advanced Hyper drive technology, both of which are present in Atlantis.