Comment by daveguy

I don't understand why it has to be non-electromagnetically interacting. Can we honestly say we should be able to resolve all the matter in a galaxy with our current sensing technologies? Why can't it just be the equivalent of non-light-emitting asteroids and planets. We can barely detect whether a star has planets in our own galaxy. And according to Wikipedia the amount of matter we would be missing in a sphere with radius from the sun to Neptune is 10^17 kg or approximately a large asteroid. Which seems like a rounding error. But my knowledge of astronomy is definitely minimal.