Dark Matter is not uniformly distributed, for example if forms big blobs that surround the visible parts of the galaxies.
Dark Energy is uniformly distributed, but the (equivalent?) density is very small, so the effect is important in the very empty space between galaxies.
'Dark Energy' is physicist code for: anti-gravitating non-stuff that we cannot see.
(i.e. Dark Energy, aka Einstein's Cosmological Constant, is a property of the vacuum.)