With lengths and breadths of hundreds of millions of light years but thicknesses of only ~20 million light years, galactic sheets (also known as great walls) are some of the largest known structures in the Universe. They consist of huge aggregates of galaxies (groups and clusters as well as isolated galaxies), and give the Universe the 'honey-comb' appearance evident in the following figures.
Large-scale structures such as sheets and galactic filaments contain millions of galaxies and span significant fractions of the observable Universe. They are thought to form through the hierarchical clustering of galaxies around primordial density fluctuations (quantum mechanical fluctuations in the density of the Universe in the very first moments following the Big Bang), and are separated by large galactic voids.
If hierarchical clustering is indeed responsible for the formation of galactic sheets, voids and filaments, it is interesting to consider that while these enormous structures reveal a great deal about the present day Universe on the largest scales, they also tell us something about processes on the very smallest scales in the very early Universe.