The hot broth of the early universe cooled unevenly. At some point, bubbles of a new phase began to pop within it, like steam bubbles in boiling water. But these bubbles weren't made of steam; they were made of a new state of matter in which particles of the dark sector clumped into larger lumps. That's how dark matter could have been born.

The authors calculated that if this transition happened at energies from thousands to millions of electron-volts (for comparison: the electron mass is half a million eV), the bubbles would have left characteristic imprints. Their walls collided, generating density ripples — compressions and rarefactions. Part of this ripple transferred to ordinary matter and influenced how the first galaxies assembled. Another part remained in the dark matter itself, giving it a clumpy texture. The most interesting part is that these two effects compete. On one hand, the ripple from the bubbles enhances clustering. On the other, the free motion of dark matter particles smears out these clumps.

Astronomical observations are already helping to impose constraints. Data from the cosmic microwave background (the afterglow of the Big Bang) and the Lyman-alpha forest (spectra of distant quasars) indicate that such a transition couldn't have been too strong or too cold. In the simplest version of the model, where all dark matter was born this way, these constraints turn into specific predictions. Future sky surveys will either see the expected ripple or rule out the model. And all this — without a single direct collision of dark matter particles in a detector.