What type of particle could have such astronomical dimensions? As it happens, physicists predict plenty of energy fields whose corresponding particles could fit the bill--namely, so-called scalar fields. Such fields pop up both in the Standard Model of particle physics and in string theory. Although experimenters have yet to identify any, theorists are sure they're out there.

Cosmologists already ascribe cosmic inflation, and perhaps the dark energy (distinct from dark matter) that is now causing cosmic acceleration, to scalar fields. In these contexts, the fields work because they are the simplest generalization of Einstein's cosmological constant. If a scalar field changes slowly, it resembles a constant, both in its fixed magnitude and in its lack of directionality; relativity theory predicts it will produce a gravitational repulsion. But if the field changes or oscillates quickly enough, it produces a gravitational attraction, just like ordinary or dark matter. Physicists posited bodies composed of scalar particles as long ago as the 1960s, and the idea was revived in the late 1980s, but it only really started to take hold four years ago.