The Large Scale Structure (LSS) of the universe refers to the patterns of galaxies and matter on scales much larger than individual galaxies or groupings of galaxies. These correlated structures can be seen up to billions of light years in length and are created and shaped by gravity. Just as gravity on smaller scales pulls together gas particles to make stars, and pulls together stars to make galaxies, it also pulls together galaxies and matter into patterns on larger scales. These patterns often contain large filaments of galaxies, and voids in between, somewhat resembling a spider web, which is why it is often referred to as ‘the cosmic web.’
Studying LSS tells astronomers about the strength of gravity in the universe. Astronomers can measure galaxies at different distances away from the Earth, which correspond to different times in the universe’s history, due to the time their light takes to reach us. We can tell that over time, gravity is attracting more and more matter together, clustering the universe further and further.
Large Scale Structure also tells us about dark energy. Most theoretical models of dark energy act to slow down this process of gravity creating large structures. Essentially, as the universe accelerates in its expansion, it takes more time for matter to come together because it must travel more distance. Studying the growth of large scale structure across time gives us information about gravity, dark energy, and how each may be changing as the Universe evolves with time.