We've built incredibly sensitive, bizarre instruments to look for them. These include vats of liquid xenon stored miles underground, and telescopes looking for dark matter particles decaying into things we can see and measure, like gamma rays. It includes the Large Hadron Collider, one of the most expensive science experiments ever built. And we haven't found them. We haven't found the WIMPs themselves, and we haven't found convincing evidence that they exist.

Except, of course, for the persistent evidence we can't ignore that says the universe is heavier than what we can see.

At this point, the unsettling feeling is growing again. Decades ago, scientists were confident about the existence of the "luminiferous aether" as a medium to carry light. Now, that's looked back on as a clumsy belief that should have been dropped far earlier than it was. Scientists persisted because they were sure that light, like sound, required a medium to move through in spite of the evidence piling up against that concept. Having been fooled once, scientists have to ask: Is dark matter the new ether?

For decades, a few rogue scientists have stood hopefully at the edge of respectability, offering their theory called Modified Newtonian Dynamics, or MOND. Essentially, it says that physics doesn't work as we know it at the largest scales. It says we've been drawing the wrong conclusions, and dark matter isn't required to explain the universe. No one has managed to develop a theory of MOND that adequately explains the universe around us, but it occasionally gains converts simply because the competing theory of dark matter has a glaring flaw: we can't find it.

Perhaps we're wrong about something in the standard model that defines how the tiniest particles in the universe behave and interact, and dark matter exists, but in a very different form than we're expecting. Or perhaps we are wrong about the laws of gravity.