Dark Matter and Dark Energy are inventions intended to solve certain bedevilling problems in cosmology and galaxy dynamics. The experimental evidence for them is somewhat circumstantial, but has generally been considered compelling. Dark matter, in particular particulate dark matter is generally considered to be on somewhat more solid ground, both because there are plausible additions to the standard model that seemingly fit the bill, but also because of its apparently crucial roles in the early universe and galaxy formation. The hypothetical particles that constitute it have remained stubbornly unobserved, however, despite extensive searches. This has led to new interest in so-called Modified Newtonian Dynamics, or MOND. This old idea tinkers with gravity itself, but has not convinced many because it's not so hot at dealing with the early universe, and, IMHO, ugly as a bullfrog flattened by a semi..
Anyway, Sabine of Backreaction has a new post out arguing that a classic bit of evidence adduced for dark matter, the Bullet Cluster, is actually a better argument for MOND. Maybe, but I don't understand the technical details.