Scientific consensuses were made to be challenged, and some scientists are challenging the consensus that dark matter — undetected, unseen, and only hinted at by the motion of galaxies — dominates the universe. It’s been the scientific consensus since the 1970s that dark matter exists and comprises more than 90% of the universe.

The mysterious dark matter that’s been called on to make sense of the ways galaxies twirl through space may not exist, if an alternative theory is right.

The surprising way galaxies rotate — as if they are much larger and heavier than they appear to be — has long implied to astronomers and astrophysicists that there is more matter out there holding things together than we see.

That unseen and unseeable matter has fallen under the catch-all term “dark matter.” These days, the most likely candidate for what makes up dark matter is some sort of weakly interacting particle that we’ve so far failed to detect.

But there is another radically different possibility: What if gravity itself doesn’t work quite the way we think? Maybe at the outer edges of galaxies where the gravitational acceleration — the g — of a galaxy is extremely small, gravity tugs just a tad bit more.

If so, that miniscule difference could be enough to cancel the need for dark matter altogether. That’s what the Modified Newtonian Dynamics (MOND) theory suggests, astronomer Stacy McGaugh explains in an article in the Aug. 3 issue of the journal Science…

Some scientists think the MOND g has already been detected by NASA monitoring the Pioneer 10 and 11 spacecrafts, which are now 40 to 60 times further from the sun than Earth.

“They show an extra acceleration directed towards the sun at about the same order of magnitude,” said McGaugh.

As these stories usually end, it’ll take more telescopes and at least 10 years of study to figure out who’s right. Hubble will get one more servicing mission, and the infrared Son of Hubble goes up circa 2013.