Astro ad nom is a logical fallacy that occurs when someone uses the vastness of the universe as an excuse for why something is true or likely to be true. The argument is that because the universe is so large and contains so many things, it is statistically improbable that something is not true.

For example, someone might argue that there must be other intelligent life in the universe because there are so many planets and galaxies. While it is true that the universe is vast and contains many things, this does not mean that it is statistically probable that there is other intelligent life. In fact, there are many reasons to believe that the conditions necessary for the development of intelligent life are very rare.

Astro ad nom is a fallacy because it relies on a false premise. The vastness of the universe does not mean that anything is possible or likely. In fact, it means that the vast majority of things are not possible or likely.