"Are there powerful technologies that require rare materials"?
To see why this is such an important question, consider what difference this makes:

If the most powerful possible entity of a given size can be made using plentiful resources, obviously so many copies can be made that it uses up a good portion of the materials in the galaxy. Individuality becomes meaningless, evolution favours spam.

However, if there is a single ultra-rare material of great importance (so rare we've never detected a sample), then a galaxy has a cap on the number of most-powerful-entities it can host. This means that making as many things as possible isn't vital for survival in interstellar competition, because you can be a mere individual and still be one of the most powerful beings in existence. However, this also means the only goal of any species would be to violently hoarde this rare resource, since it's the only thing that has nonzero value to an advanced race (and thus can't be bought)

If there are multiple ultra-rare materials, it's much the same, except with one crucial difference: It means there are multiple things with nonzero value, thus it's possible to have goals other than violent hoarding. Things like trade are possible.