When I consider Thy heavens, the work of Thy fingers, ...which Thou hast ordained;.... Psalm 8:3
"The amount of visible matter in our galaxy, all the stars, gas clouds,planets, asteroids, and every bright shining object we can detect, accounts for less than 15% of what would be needed to produce these observed rotation speeds. Think about that for a moment:
Everything we can see, photograph, and study in our galaxy represents a tiny fraction of what must actually be there."
SleepyPhysicist