It seems like you’re asking about solids found on Earth, which rules out interstellar objects like black holes or neutron stars.
So the honor goes to this rare, beautiful, blue-tinged metal:
That’s Osmium, the densest element in the periodic table.
These elements have the highest density:
- Osmium, 22.6 g/cc
- Iridium, 22.4 g/cc
- Platinum, 21.45 g/cc
- Rhenium, 21.2 g/cc
- Uranium, 20.2 g/cc
Since those are the highest density elements, they are the highest density solids possible when constructing a solid out of whole atoms. In order to get something more dense, you have to use the space inside atoms, which gets us to a whole different level of dense, man-made materials.
In 2010, scientists at the Brookhaven National Laboratory claim to have produced a state of matter that was only hypothetical until that time, producing a quark-gluon plasma (or “quark soup”), which can exist only at extremely high temperature and pressure. This material is thought to be the densest ever to have existed on Earth. However, this has two strikes against it when competing as an answer to your question:
It’s man made, and it seems as if you’re asking about naturally occurring solids.
Quark Soup is considered a different state of matter, and, hence, not a solid.
Therefore, I think pure Osmium is the definitive answer to your question.
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