Or the reason I think that an infinite universe exists outside of our Big Bang is because we can see the effects of all that extra gravity in the behavior of our own big bang galaxies, not to mention that the JWT has actually seen galaxies older than our big bang and hence born from BBNs outside of our big bang.

That a "fictitious" illusion of dark matter can arise from actual real very-far-away matter has been calculated in the work of A. Carati et al. (2009)
https://astronomy.com/news/2023/04/einstein-rings-inch-us-closer-to-solving-dark-matter-debate