But the gravitational pull of galaxy clusters and dark matter—the metropolises of the universe—tweak, twist, and wiggle that relic radiation. This phenomenon is called gravitational lensing, and for anyone looking through a telescope, it creates a distorted picture of the cosmos. Yet it presents a boon for astrophysicists, because those distortions are actually clues about how the universe developed after its infant years.
https://www.wired.com/story/is-cosmology-broken-atacama-telescope/