The Negative Zone is parallel universe made completely of anti-matter where cosmic beings dwell.
The only entrance and exit is through the portal that leads to the Negative Zone where the prison stands. It's guarded by elite Mandoids somewhere inside the Baxter Building.
The Negative Zone is home to creatures that serve Annihulus. Galactus was banished there in order to save Earth and so that he could feast on the endless anti-matter.