What if dark matter didn’t exist?
Sure, scientists have never observed it, but they believe it exists because of apparent gravitational effects. But what if our current understanding of gravity was just plain wrong? The question has been raised over the last several decades, but typically when a proposed modified Newtonian dynamics (MOND) theory is put forth it has too big a blindspot to be taken seriously in the scientific community.
For more, check out this Salon/MSN story.