But we know they "spin too fast", by measuring the amount of observable matter and then measuring the "rotation curve" of the galaxy. However, the basic point t

But we know they "spin too fast", by measuring the amount of observable matter and then measuring the "rotation curve" of the galaxy. However, the basic point that this is not half the dark matter, is correct, (based on our current understanding). Even if we can't see ordinary matter, we should be able to see its effect in some way other than gravitational interaction. Dark matter is only seen by its gravitational interaction. It doesn't interact electromagnetically.