[InclinedPlane]

Dark matter is a weakly interacting massive particle which hasn't been detected directly yet. This is not a terribly controversial theory because we already know of other particles with very similar behavior (neutrinos) and we also know for a fact that our theories of particle physics are incomplete. Moreover, the most straightforward "improvements" of our current theories of particle physics (supersymmetry, for example) would all necessarily involve the existence of previously undiscovered particles. Additionally, the conditions by which such "new" particles would be created (high energy densities) are precisely the conditions which would occur during the early Big Bang, leading to a very natural process of formation of a "cosmic dark matter background" (just as there exists a cosmic neutrino background and a cosmic microwave background) as a relic of the Big Bang, which unlike the other background relics would evolve in structure significantly after the Big Bang, responding to the evolution of baryonic matter (into clumps which became galaxies and galaxy clusters). In short, it's not very far of a leap from everything we already know about physics and cosmological history, and it matches the observational evidence remarkably well, which is why it is the accepted theory in astronomy and cosmology.

Dark energy is less well studied but the idea of vacuum energy is not a new one. We don't understand why vacuum energy is not zero but instead a very tiny number, but it's only a little surprising. It is, after all, fundamentally a theory that is a century old, dating back to Einstein's work with the cosmological constant.