Late in the 20th century, Saul Perlmutter discovered that the expansion of the Universe was accelerating. This invisible force was named dark energy. Standard theories required “ghosts”—particles with negative energy that break the foundations of physics.

A new model does without them: it’s all like a compressed spring. Shortly after inflation, a microscopic axion (a particle predicted by Helen Quinn) got pinched inside a dense cloud of dark pions — the building blocks of dark matter, discovered by Vera Rubin. As space stretched, the cloud thinned (redshift increased), and half a billion years ago, the pressure eased: the axion-spring broke free and is now pushing galaxies apart at an accelerating pace.

The model builds on laws akin to the Standard Model of particles, and computer simulations agree with the cosmic microwave background. A surprise: all the power of dark energy fits into a tiny speck of space, and the dark pion’s mass is thousands of times lighter than an electron. If this holds true, dark pions will reveal themselves for the first time not only through gravity but also in the accelerating expansion.