Is there a name for a point at the edge of the observable universe at which there is no visible matter, that appears completely dark?
I'm thinking something like "empty space", but that could equally apply to an empty void between galaxies, which isn't quite what I'm looking for.
So it would refer to a theoretical line in space from the observer to the edge of the universe which contains no visible matter. Is there a term for this?
There is no name for this.
As we look further in space, we also look further back in time. The "edge of the observable universe" is in the early universe. At an early time in the evolution of the universe, the whole of space was so hot and dense that it was opaque. In whichever direction we look, we can't look back further than this. The is called the cosmic microwave background. There is no direction in which we can look in which we don't see the cosmic microwave background. There is no direction that contains no visible matter.
There are directions that contain no galaxies or other more nearby objects, but there is no name for these directions.
There is a name for the region of space (or rather the period of time) that is beyond the furthest galaxies, but closer (or more recent) than the cosmic microwave background. This is the "Dark ages".
I think you're asking two (related) questions: First you're asking about the "boundary" of the observable Universe, i.e. the points in space separating the observable Universe from the rest of the Universe. Second, you're asking about a line going from us to this boundary.
The observable Universe is bounded by the particle horizon, which is defined as the spherical, non-physical shell centered on our position, and with a radius equal to the distance light has had the time to travel since the Big Bang.
This distance, which is your second question, is simply "the radius $R$ of the Universe", and may be calculated by integrating the scale factor $a(t)$ from a time $t=0$ to $t=[\mathrm{today}]$: $$ R = \int_0^{13.8\,\mathrm{Gyr}}\!\!\!\!\!\!\frac{c}{a(t')}dt'. $$ The scale factor depends on the expansion rate $H_0$ and the density parameters $\Omega_{\mathrm{m},\Lambda,\gamma,k}$ of the various components of the Universe. For the observed values of Planck Collaboration et al. (2020), this integral amounts to $46.2$ billion lightyears.
Wouldn't the edge of the universe just be the location where there is no matter or energy beyond. Assuming light has reached the farthest from the big bang point, wouldn't the edge just be at the farthest locations where light has traveled. Beyond the edge would just be the same as the empty space all around us (locations where there is no matter or energy) except that there would be no matter or energy within an agreed upon distance. Can empty space have a distance, yes, but that doesn't mean it is part of the universe.
The observable universe is merely how far we can see with current telescope technology.
Space itself is endless. Oh, there's no end of people here who will aggressively argue this. But empty space, by definition, cannot have an end. It can only be infinite. Literally infinite.
There could very well be an edge in the sense that there's no more matter, where stars, planets and galaxies etc simply tapers off. After that, it would be endless black space.
