The observation that our Universe is accelerating must be one of the most striking breakthroughs of the twentieth century. Since its discovery in the 1990’s theorists have been trying to determine the nature of whatever is causing this acceleration. It is dubbed dark energy principally because we do not know what it is. Possibilities include Einstein’s famous cosmological constant, but if it is the cause we do not understand why it has the value it has. On the other hand it could be dynamical in origin perhaps signifying a time varying contribution of a fundamental scalar field, but such a field would generically bring with it a fifth force, whose existence is tightly constrained by solar system tests of General Relativity (basically we have no evidence for its existence on solar system scales). Such models require that on such scales the fifth force is screened from us, but we could be experiencing the force on cosmological scales. In this colloquium I will review the current state of the Universe on large scales, I will discuss the problems associated with the cosmological constant from a theoretical standpoint, accepting that it probably remains the best fit to the cosmological data, and I will discuss the prospects of detecting these screened scalar fields if they exist, using physics ranging from atom interferometry through to galaxy rotation curves. Finally we will discuss the impact of the recent exciting development of multi-messenger astronomy where the coincident detection of photons and gravitational waves from colliding neutron stars have conspired to place tight constraints on a plethora of modified gravity models.