we can be fairly certain about distances when we can measure them by parallax, to triangulate them, but at a certain distance the parallax differential is not even computable (think like doing math with numbers that require exabytes to represent the precision). let alone the device measuring the angle, that's probably way more limited.

so, how do you anyway decide how far away something is just by the light it sends back to you, if you don't have parallax? the motion of the objects would serve to help bring some ratio to it but even that would fade to zero beyond a few galaxies deep.

this whole train of thought has totally broken my brain though. since we know that light travels slower the more matter is near it, the speed of light MUST be higher between galaxies.