In the 1920s, Edwin Hubble noticed galaxies were moving apart — thus discovering the expansion of the Universe. Decades later, Adam Riess and colleagues, measuring the speed with supernova explosions, found that the Universe expands faster than predicted by measurements from its “baby pictures” — the afterglow of the Big Bang. This mismatch was dubbed the Hubble tension.
A new theory explains it using a kitchen metaphor. The Universe is like raisin dough. From the heat lingering after the initial oven burst, you could calculate how fast the raisins should move apart. But they're moving faster — as if yeast woke up in the dough billions of years later. That's exactly the hidden chameleon field scientists propose. It also adds invisible mass — like a cloud of flour weighing down the dough and keeping galaxies from falling apart.
Thus dark energy and dark matter turn out to be two sides of a single force, and spacetime curvature on large scales stops contradicting the early Universe.
If the Universe had expanded slower in the past, gravity would have squashed everything back; if faster, galaxies could never have formed. We exist on a knife's edge.