Quark-gluon plasma is the densest form of matter ever observed. It was created during an experiment at the Large Hadron collider in 2011. According to National Geographic, it is assumed that the matter making up a black hole is more dense, but this has never been observed or measured.
Quark-gluon plasma is more than 100,000 times hotter than the inside of the sun and has a higher density than the matter of a neutron star. One cubic centimeter of this plasma would weigh 40 billion tons. For comparison, the most dense naturally occurring elements known to man, osmium and iridium, weigh about 23 grams per cubic centimeter.