When considering issues in philosophy, it's often useful to keep track of the long list of ideas whose veracity can neither be verified nor denied. Indeed, these can often have quite far reaching implications in unexpected places.
The idea that the universe was created last Thursday such that it appears exactly the way it would have appeared through the long process of physical evolution in the way that is currently understood.
The idea that everything we observe is actually the result of a computer simulation and that we in turn are merely facets of that simulation. This scenario roughly appears in the movie "The Matrix," although the actual existence of one's physical body in some other world is not required.
The idea that you're the only person currently alive; the others possibly being a figment of your imagination. If this were true, morality would be considerably simplified with rights, pleasure/pain, etc. all becoming moot.
Many worlds hypothesis:
The notion that the universe can branch off into separate autonomous universes giving rise to some sort of multiverse. The most common and most plausible variant of this is the many worlds interpretation of quantum mechanics in which every quantum mechanically possible outcome occurs in its own universe instead of a wavefunction collapse in just a single universe.
The latter of these is probably the most interesting for the rather spectacular way in which it undermines the efforts of antinatalists and other would be good-doers. Implicit in many discussions of morality is an idea of causality that can't be proven. Typically, one thinks in terms of cause and effect but in the MWI case we may have a single cause that can be linked with many mutually exclusive possibilities had these occurred in a single universe. The result of this is that statements like "if I shoot someone, they will die" are no longer meaningful. Rather, it's "if I shoot someone, they will both live and die no matter how good a marksman I am or how I shoot them with what etc." The notion of free will likewise becomes incoherent for then people no longer have the option to choose between two alternatives as both end up occurring regardless. Note: it's possible to interject here that the two outcomes are not actually associated with the same person i.e. that as people branch off into different universes, they become distinct. I won't pursue this line here.
Many quantum physicists regard the many worlds interpretation as a serious possibility which is clearly bad news for antinatalists. If the MWI is correct, then there are possibly an infinite number of people currently existing and more will continue to come into existence regardless of what anyone does or can do.
In spite of this, one can still simply act as though the world exists as it appears, as though others exist and as though actions can make a difference just in case they actually do in an attempt mitigate potential harms. Indeed, as these theories are unfalsifiable and unprovable, they are more things to be kept in the back of one's mind to serve as counter examples to other arguments.
I'll add more interesting unfalsifiable theories to this list as I think of them.