Consistency with quantum gravity can impose non-trivial "swampland" constraints at low energies, even if the Planck scale is at very high energy. In this talk, I will review some of the most important swampland constraints and their relations. In particular, the Distance conjecture implies the presence of infinite towers of states becoming massless at the weak coupling/large field limits, which poses constraints on inflationary models. This has been extensively tested in string theory compactifications. In the presence of a scalar potential, the mass of the tower of states seems to be correlated to the value of the vacuum energy in all known stringy examples. Hence, the presence of a light tower could potentially explain the smallness of the vacuum energy in our universe. I will show how experimental constraints force this tower to correspond to a KK tower (of mass of order neutrino scale) of a single extra mesoscopic dimension of order 10^{-6) m, which we denote as the Dark Dimension.
