Some researchers are trying to narrow it down:
Large spiral galaxies are one galaxy type that researchers think may be good for developing life. Our own planet is the only known example of life arising in such a galaxy, but spirals pack relatively high amounts of the heavy elements needed to form rocky planets.
However, life in a spiral galaxy can have its downsides, too. These galaxies form new stars more actively and have more dangerous cosmic events, like supernova explosions, compared to other galaxies. Those kinds of disasters can spew harmful radiation into nearby space and potentially destroy planets’ biospheres.
So, perhaps galaxies with less active star formation, and fewer cosmic explosions, might be calmer, safer places that allow planets more time to develop life.Erika K. Carlson, “Which Galaxies are Best Suited for the Evolution of Alien Life?” at Discover Magazine
When the researchers tested the thesis by studying 100,000 simulated galaxies, they found that small or dwarf galaxies with comparatively abundant heavy elements offered the best chance.
Some of us would settle for a single fossil bacterium on Mars and forget all the theories.
See also: Once Upon A Time, Venus (Might Have) Had Life, Say Researchers