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Inversely, there's plenty of things where we know how they work but we don't know what they are, such as gravity. In this case, people know how the universe functions with the HB particle but they don't know what it is. But we still know, at least, that the HB particle is a theoretical particle imagined for particles to acquire non-vanishing masses.
In what sense is that true about gravity? Gravity is defined a priori as the phenomenon of objects with mass experiencing an apparent force. The cause is the bending of spacetime, but this understanding (of 'how it works', I suppose) is incomplete because it's inconsistent with other theories.