From what I understand, they found some amount of hydrogen in infrared and noticed that their observations deviatated tremendously from their theories of gravity.
Then they introduced this inordinate amount of "dark matter" as an inordinately large fudge factor. In this case, the fudge matter had to be made more abundant than the observable matter in order to force-fit the square peg of their observations into the round hole of their theories.
--Astro
