According to our current understanding of black holes, the dimensions of a supermassive singularity at the center of a black hole are considered to be infinitesimally small, often described as a singularity of zero size. A singularity is a point of infinite density and curvature where the laws of physics, as we currently understand them, break down.
However, it's important to note that the singularity itself is hidden from direct observation. The concept of an event horizon comes into play here. The event horizon is the boundary around a black hole beyond which nothing, including light, can escape its gravitational pull. It effectively acts as a one-way membrane. The event horizon hides the singularity from our direct view.
From our perspective on Earth, if we could somehow directly observe a supermassive black hole, what we would see is the black hole's event horizon, which appears as a dark region against the background of stars and galaxies. This dark region is caused by the immense gravitational pull of the black hole preventing light from escaping.
The appearance of a supermassive black hole's event horizon would depend on various factors, such as the black hole's mass, spin, and the surrounding matter. In recent years, scientists have obtained images of the event horizon of the supermassive black hole at the center of the galaxy M87 using the Event Horizon Telescope. The observed image showed a bright ring surrounding a dark central region, corresponding to the black hole's event horizon.
It's important to reiterate that what we observe is not the singularity itself but rather the effects of its extreme gravitational field on the surrounding spacetime and matter. Directly observing the singularity itself remains beyond our current technological capabilities and the reach of our understanding of physics.