Dark matter – the invisible substance whose gravity shapes galaxies – may have begun its life very differently than commonly thought. In a peer-reviewed study published in Physical Review Letters, physicists Guanming Liang and Robert Caldwell (Dartmouth College) propose that dark matter originally consisted of massless, relativistic particles in the early universe. These particles, moving at near light-speed, later condensed and suddenly gained mass as the universe cooled. In effect, dark matter would have "gone from being light to being lumps," as Caldwell explains.

Background: The Dark Matter Mystery

Dark matter is inferred from its gravitational pull – it makes up roughly 85% of all matter in the universe – but it has never been directly seen. For decades, physicists have searched for heavy, slow-moving dark-matter particles (so-called "cold dark matter") such as WIMPs or axions. Those leading candidates have so far eluded detection. The lack of evidence for WIMPs, axions, or even primordial black holes opens the door to more exotic ideas. Liang and Caldwell's new model is one such alternative: it draws on known physics (superconductivity analogies) to explain dark matter's origins.

The New Proposal: From Massless to Massive

In Liang and Caldwell's scenario, the early universe was filled with extremely energetic, light-speed particles – similar to photons – governed by a dark-sector version of particle physics. These "dark fermions" initially behaved like radiation. As the universe expanded and cooled, however, the particles began to pair up via their quantum spins (much like how opposite magnetic poles attract). The model envisions a sudden phase transition: paired particles lose energy and condense into a cold, heavy state, abruptly gaining mass. Caldwell likens it to "steam rapidly cooling into water," where the energy plummet bridges an early high-energy state and the low-energy condensate we now call dark matter.

• Massless beginnings: Early "dark particles" moved at relativistic speeds like light.
• Phase transition: As the cosmos cooled, pairs of these particles condensed (in an analogy to Cooper pairing in superconductivity), causing them to gain mass suddenly.
• Cold dark matter outcome: The result is a population of slow, heavy particle pairs – effectively cold dark matter – consistent with cosmological observations.
• Testable signature: This history would leave a distinct imprint on the cosmic microwave background (CMB) radiation, making the theory testable by current and next-generation CMB experiments (e.g. Simons Observatory, CMB Stage-4).

Crucially, the authors note that unlike many speculative ideas, this mechanism yields concrete predictions. The dark matter condensate would follow a time-varying equation of state and produce unique fluctuations in the CMB. If observed, such signatures could confirm the theory.

Superconductivity Analogy and Physics Details

Liang and Caldwell's work draws on an analogy with superconductivity. In superconductors, electrons form Cooper pairs at low temperature, allowing them to conduct electricity without resistance. Similarly, the new theory posits interacting fermions in a dark sector that form paired condensates below a critical temperature. These hypothetical fermions can be described by a well-known particle physics framework (the Nambu–Jona-Lasinio model) and behave like ordinary radiation at first. When the temperature falls, a second-order phase transition causes pairs to bind and become massive.

Robert Caldwell notes that this picture is "totally antithetical to what dark matter is thought to be – [which is] cold lumps" – yet it naturally explains how dark matter could emerge from a hot early universe. The phase transition also elegantly accounts for the large drop in overall energy density from the Big Bang to today, while simultaneously increasing mass density in cold matter. As Liang comments, the model is "rather simplistic" and builds only on physics we already know, without exotic new assumptions.

Implications and Future Tests

If validated, this theory would reshape our understanding of cosmic history. It suggests that dark matter was forged in the primordial fireball in a process akin to condensation, rather than existing as always-massive particles. This approach also links dark matter to other fundamental questions: for example, the model requires a slight asymmetry between particle chiralities, which intriguingly connects to the matter–antimatter imbalance in the universe.

Most importantly, the theory is observationally grounded. The authors argue that existing and planned CMB data could confirm or refute their ideas. As Rini's summary notes, next-generation CMB surveys should reach the precision needed to "vet" the model. Caldwell and Liang hope that data from the Simons Observatory and CMB-S4 will soon be used to search for the predicted signature. Until then, the proposal stands as a bold, peer-reviewed hypothesis grounded in known physics, awaiting further scrutiny by the community.