While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Image credit: Illustris Collaboration / Illustris Simulation.

Why are dark matter and modified gravity in such conflict?

Something isn’t right about the Universe without something extra. So why can’t scientists agree on what that “extra” thing is?

“All we know so far is what doesn’t work.” -Richard Feynman

The laws of gravity are some of the most well-established and best-tested physical laws of all-time. If you made an observation of a massive object in space — a planet, star, galaxy or something even bigger — that didn’t appear to line up with what gravitation predicted, you’d be crazy not to double-and-triple-check what you saw. But every once in a while, either our laws of physics or our understanding of what’s in the Universe turn out to be incomplete, and it’s up to us to figure out the way forward. Right now, a huge academic battle is taking place between two camps seeking to fix the gravitational problems of the Universe: the dark matter camp and the modified gravity camp. This is a battle that’s played out before, with each side having historical victories to point to.

Conceptual drawing of a solar system containing numerous planets, all obeying the laws of gravity. Image credit: NASA/Tim Pyle.

In 1781, the planet Uranus was discovered. The first large Solar System object ever…