 The physics horizon poses a challenge because one particularly powerful type of evidence—direct experimental detection or observation, with no dependence on cosmological assumptions—is unavailable for the physics relevant at earliest times (before inflation, and indeed even for baryosynthesis after inflation). Yet this does not imply that competing theories, such as dark matter vs. modified gravity, should be given equal credence. The case in favor of dark matter draws on diverse phenomena, and it has been difficult to produce a compelling modified theory of gravity, consistent with GR, that captures the full range of phenomena as an alternative to dark matter. Cosmology typically demands a more intricate assessment of background assumptions, and the degree of independence of different tests, in evaluating proposed extensions of the core theories. Yet this evidence may still be sufficiently strong, in the sense discussed more fully in  §5  below, to justify new physics.