A balloon inflates on its own, even though no one is blowing it up. That's roughly how the accelerated expansion of the universe looks, and to explain it, scientists usually invoke dark energy, an invisible and poorly understood force. But what if the balloon expands because new air molecules are continuously appearing inside it? This work explores exactly that mechanism: dark matter particles (the invisible substance that makes up most of the mass of galaxies) are born directly from the vacuum and, along the way, exchange energy with another cosmic fluid.

The authors examined six different scenarios for this exchange — something like six recipes for how energy flows between two fluids. It turns out that a stable state, where both fluids coexist and expansion accelerates, arises only in three cases: when the exchange is controlled by the density of dark matter and energy flows from it to the second fluid. If the exchange depends on the properties of the second fluid or on mixed parameters, the universe eventually reaches a different outcome — domination by dark matter, but still with acceleration.

This approach is reminiscent of trying to explain a car's acceleration not by a mysterious tailwind, but by the fuel in the tank synthesizing itself. The key takeaway: dark energy might not be a separate entity, but rather a side effect of particle creation and energy redistribution in the cosmos. This doesn't eliminate the mysteries, but it provides a new tool for solving them — without invoking invisible entities.