In the '60s, quasars and the cosmic microwave background were the rage; in the '70s, ultrafast pulsars made the headlines. Gravitational lensing that bent space and distorted background galaxies was "in" just a few years ago. Now it's dark energy and dark matter.
None of these crazes were small potatoes, but it's hard to top today's hot subjects: Dark energy - a sort of antigravity force that makes up 75 percent of the universe - is too weird and too new; so let's stick right now with the theoretically easier dark matter, which makes up almost one-quarter of the Cosmos.
The problem is easy to visualize, even though it seemed to dawn on most astronomers very slowly, like the person searching for their parked car only to fathom, finally, that it's been stolen. It was decades before textbooks caught up with pioneers like Fritz Zwicky, who discovered dark matter way back in 1933. He'd correctly determined that the motions of neighboring galaxies are so fast that they ought to have escaped the confines of our local group of galaxies long ago. But they haven't, so some sort of unseen gravitational epoxy is holding things together.
Within our own galaxy, too, we've long realized that it rotates as if it has six times more matter than can possibly be accounted for by all the stars, black holes and anything else we can think of. The Milky Way's familiar bright pinwheel, like a ship in a bottle, is apparently immersed in and surrounded by a huge halo of unseen material.
Star motions in other galaxies, and interactions between galaxies, show that hidden mass is a recurring theme throughout the Cosmos. The best bet is that it's some sort of exotic, massive, subatomic-sized particle that does not interact with the atoms that make up our bodies, our planet and the stars. It neither emits light nor reflects it, because it obviously has no electrons, nor influences them. It's like a parallel dimension: stuff that has its own independent unseen existence - and it could be right here, too, in our rooms, perhaps forming its own invisible structures that cannot even be felt. Maybe there are even exotic dark-matter life forms.
Unless...there's one way out: Dark matter needn't exist at all if gravity itself behaves strangely at large distances. Maybe it's gravity that we haven't fully understood. Perhaps its power doesn't diminish in the same simple way (with the square of distance) that operates within smaller scales. Maybe that's why the now-distant departing Voyager spacecraft are both behaving oddly, their speeds failing to match calculations.
Which is it? That we're wrong once again, this time about the most important of the four fundamental forces? Or, instead, that our very bodies - as well as everything that we've ever observed - are composed of minority material, unrepresentative of the universe?
Which do we choose: the improbable or the exotic?