Classical black holes, regions of spacetime with gravity so strong that even light cannot escape, according to general relativity, hide a singularity at their center: a point of infinite density and curvature where the known laws of physics break down. However, there are theoretical models in which the singularity is replaced by a dense but finite-sized core of exotic matter. To an outside observer, such objects are almost indistinguishable from classical black holes, so astrophysicists are developing methods to "look" inside.

One approach is analyzing the weak bending of light from distant stars passing near a massive object. The gravitational field acts as a lens, distorting the paths of rays, but this technique is like trying to feel the shape of an object with your palm: it gives only a blurry impression. Much more informative are direct images of the black hole's shadow — a dark silhouette against the glowing accretion disk. By observing the size and shape of the shadow, one can estimate the parameters of the central core with high precision.

However, even a sharp shadow does not always reveal the secret: the geometry of spacetime can mask microscopic details, much like a hard shell prevents you from tapping the filling of a nut. To "hear" the internal structure, whether a dull thud or a clear ring, scientists turn to gravitational waves: ripples in the very fabric of space, born when a black hole oscillates. Analyzing how these waves interact with the horizon, as well as measuring time delays of light near it, can reveal the nature of the core. Another unexpected clue is the behavior of infalling gas. When the accretion regime changes, as matter transitions from turbulent to laminar flow or vice versa, the observed brightness rearranges: the familiar rhythm of flares seems to falter, and this anomaly could be the signature of a regular core.

The combination of such signals will allow us to determine whether behind the horizon lies a bottomless singularity or a bridge to another region of spacetime. Remarkably, the first model of a regular black hole, without a singularity, was proposed back in 1968, long before the era of direct observations. Today, thanks to modern telescopes and gravitational wave detectors, this hypothesis can finally be put to the experimental test.