According to our current understanding of physics, astrophysics, and cosmology, most of the universe is made of “dark”, yet-to-be-detected, components. As illustrated above, about three fourth of the cosmic energy budget is accounted for by Dark Energy and one fifth by Dark Matter. Ordinary matter also called baryons (mostly hydrogen and helium) represents only about one twentieth of the budget. But still, most of it (roughly ninety percents) lies in a diffuse and hard to detect warm intergalactic plasma. Heavier elements also called metals (mostly oxygen and carbon) take various forms (white dwarfs, neutron stars, black holes, dust, planets...) and we don’t even know how much are hidden in the intergalactic medium. This may be why, I am involved in this “dark business”, trying to shed light on these various “dark” components. Where are they? What are they? What are their properties? Do they really exist or did we miss something (new physics...)? And more important how do these components evolve and interact during the course of large scale structure formation (galaxies, cluster of galaxies, filament, voids...)?