Ordinary black holes hide a point of infinite density inside — a singularity, where physics breaks down. But some models replace it with a dense, yet finite core. From the outside, they're almost indistinguishable, and astrophysicists are looking for a way to probe the truth. Weak light bending from distant stars — like a palm feeling a shape — gives only a blurry picture. Direct images of the shadow, however, are a clear silhouette, and from it we can measure the core with surprising accuracy.

But even a sharp shadow doesn't always reveal the secret: geometry hides microscopic details, just as a shell prevents you from tapping out the filling. To hear the ring or the thud, scientists catch gravitational waves — ripples in space from the hole's vibrations — and analyze light delays near the horizon. An unexpected fact: in infalling gas, when the flow regime changes, the brightness rearranges itself, as if a familiar rhythm stumbles — and gives away the core's secret.

These signals will help us understand whether there's an abyss or a bridge. The first model of a regular black hole appeared in 1968, long before any images, and now it can be tested.