Imagine a huge layered cake, where the layers of dough are space and the filling is a special region resembling an invisible wall. This wall is not made of bricks but of energy, and it could be as thick as the entire universe. If such a wall exists, it should stretch the dough layers differently: those lying along it spread apart faster, while those across it spread slower.
This is exactly the scenario the authors tested. They took a model where dark energy, the mysterious force making the universe expand faster and faster, is confined to a giant flat structure, a domain wall. If we were inside such a wall, we would notice that light from distant stars arrives distorted, and the expansion of the cosmos itself would depend on direction.
But when scientists compared the calculations with real data, the picture did not hold up. The Planck satellite measured the temperature of the cosmic microwave background across the entire sky with great precision, and no trace of a wall was found. Its contribution to expansion would have to be a hundred thousand times smaller than that of ordinary dark energy so as not to spoil the neat picture. An additional check using distant type Ia supernovae from the Pantheon+ and DESY5 catalogs confirmed that the universe expands almost uniformly in all directions, just as the standard cosmological model says.
The layered cake turned out to be flat — no giant wall inside. Perhaps dark energy is not hidden in one big structure but is spread everywhere uniformly, like an invisible syrup soaking all the dough.
The cosmic microwave background used for the test is the oldest thing we can see. It emerged just 380,000 years after the Big Bang, when the universe became transparent to light.