Full Idea
There are two types of mass: gravitational mass quantifies how strongly an object feels gravity, while inertial mass quantifies an object's resistance to acceleration. There proven equality is at the heart of General Relativity.
Gist of Idea
Gravitional mass turns out to be the same as inertial mass
Source
New Scientist writers (Why the Universe Exists [2017], 05)
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.101
A Reaction
It had never occurred to me that these two values might come apart. Doesn't their identical values demonstrate that they are in fact the same thing? Sounds like Hesperus/Phosphorus to me. The book calls it 'mysterious'.