Galaxies/clusters are structured as filaments(lines) and voids (bubbles). This structure is Three Dimensional. The picture above is color coded for distance. Things nearby are purple-blue, far are green-yellow and the most distant structures are red. The Milky Way is pictured at the center and blocks our view along its plane (called zone of avoidance).

How do we get distances to these Galaxies?
Hubble noticed in 1921 that all external galaxies were receding from us. The most distant galaxies had the greatest velocity moving away from us. This Hubble Law is an effect of our expanding universe (more on that when we talk about cosmology). This relationship was one of the most important discoveries of the 20th century for two reasons: 1) It demonstrated the expansion of the Universe; 2) Once calibrated, it allows distances of galaxies to be derived based on a Doppler Shifted spectrum.

Because the light from the most distant galaxies/clusters took millions or billions of years to get to us, we are actually looking back in time. We can see how galaxies came to form and create the large scale structure seen today.