Black holes are created by the gravitational collapse of massive stars.
But in theory, black holes can also be created by any mass at all that is confined within a small enough volume of space, defined by the Schwarzschild radius.
If you keep adding more and more mass, eventually a black hole is formed, where you cannot see the matter anymore (although you can still detect it in various ways)
So, this made me realize that there must be some kind of intermediate step, between a visible object with high gravitational lensing, and a black hole.
Would the object appear to slowly get darker and darker as more mass gets added, as the escape velocity gets closer and closer to the speed of light?
This would be an interesting object to place in my universe, and an interesting discovery. For bonus points, what would be easier to detect? This theoretical object, or a black hole? (since this object is not a star, would it not radiate?)
If this theory is completely wrong, please put that in the answer and explain it. I am not an expert in this field of science.
Worldbuilding context: It's just an interesting celestial object whose discovery I would put in my story and its history books and whatnot.