They didn't say it existed.
They just said the existence of Dark Matter seems to be more likely than non-uniformity of physics throughout the universe.
Dark matter is a label for "whatever it is that causes the extra gravitation observed across multiple scales in the universe without also causing other indications of matter interaction like photon emission."
These are easy-to-hold handles for the collection of observations for phenomena we know to exist. We don't know the cause, but we know these things are occurring in the universe because we have demonstrated their existence through repeated observation.
Saying the laws of physics may be different in different parts of the universe is a statement that so far can't be substantiated, and we've been trying for over a hundred years now, is far more problematic. That's a dire option that we don't desire to pursue unless we're absolutely forced to - and we're not being forced to.
Between the two options we're going to stick with dark matter/dark energy until evidence suggests otherwise, or until a better, and simpler, idea comes along.
Claiming physics isn't uniform throughout the universe is more problematic than positing the existence of dark matter/dark energy. Lacking any evidence for either position, the principle of Occam's Razor informs us to stick with dark matter/dark energy.
One of the purposes of exercises such as this is to get people thinking of new ways to falsify existing models. So far, dark matter/dark energy are in remarkable alignment with observations. Falsifying either of these ideas would be huge.