A &quot;competing theory&quot; would first have to match the GR predictions in all the other regimes where it&#x27;s already been tested. But doing that is an extremely strong constraint on a theory, to the point where the only theory that can meet it is GR itself. Physicists know this because alternative theories to GR have been constructed and tested, and they have all failed. See, for example, here:<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Alternatives_to_general_relativity#Results_of_testing_theories" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Alternatives_to_general_relati...</a>