The highest possible density of an object is a concept that depends on the physics involved. In the realm of astrophysics, there is a theoretical upper limit on the density of matter known as the Chandrasekhar limit. This limit is approximately 2.7 x 10^17 kilograms per cubic meter (kg/m³). It represents the maximum density that a white dwarf, a type of collapsed star, can attain before it undergoes further collapse or transformation.
If an object were to reach such extreme densities, it would experience tremendous gravitational forces. At densities near the Chandrasekhar limit, the object would likely be a white dwarf—a stellar remnant composed primarily of degenerate matter, where the electrons are squeezed so close together that they occupy quantum states prohibited by the Pauli exclusion principle. The immense gravitational forces would keep the matter compressed and prevent further collapse.
In terms of appearance, a white dwarf at this extreme density would likely be a hot, dense object emitting intense radiation, typically in the form of visible light or X-rays. However, it's important to note that this is a theoretical limit, and it is not yet fully understood what would happen if an object were to exceed the Chandrasekhar limit or if other exotic forms of matter exist that can achieve even higher densities.
It's worth mentioning that there are other highly dense objects in the universe, such as neutron stars and black holes. Neutron stars, which are formed from the remnants of massive stars, can have densities on the order of 10^17 to 10^18 kg/m³. Black holes, on the other hand, are regions of space where the gravitational forces are so intense that nothing, not even light, can escape their gravitational pull. The density at the singularity of a black hole is thought to be infinitely high, but our current understanding of physics breaks down at that point, so it's challenging to define its density precisely.