Well since the triangle shapes perfectly form the outer "nebula" shape, I don't think there was any mask involved. I think the triangles were just placed until that shape was formed. The part that I don't understand is has a gradient kind of feel overall. You could do the manually as the last guy said with blending modes etc. but it seems to me that it has a gradient overall applied...or something.
There are about 3 million different ways to do the same thing in Illustrator. I think 12dylan34 laid out a very simple two-step plan. You could reverse the steps and do the triangles first, then make the mask based on those triangles. Then it's just a matter of setting transparency and blend modes.
I don't think this artwork is as complicated as it seems. It's very nice, but not that hard.