An array of things, that are assumed to exist, but lack empirical evidence. For instance:
- The mind - and certainly the minds of others. Science hasn't detected the mind. Only electrical signals in the brain can be detected, but that is not the same thing as the mind. - Gravity; no one knows what makes gravity or any gravitational particles;
- Dark matter, dark energy - the mysterious particles supposed to make more than 90% of the universe
- Blackholes; certain cosmic space wherein gravity is supposed to be extraordinarily high pulling force - but no one has seen it;
- Multi-universes; a billion mysterious universes similar to our universe, explaining supposedly very fine tuning in the big bang process - which will never be backed up by empirical evidence (by the way this is the final current view in physics)
- Time; we all believe time that exists - but no one knows what actually time means and does it exist like physical things. Some physicists perceive time as entropy, but again this is very wishful thinking on their part, not backed up by empirical evidence;
- Spacetime; no one can accurately measure position and velocity of a particle and this is very fundamental problem in physics. And if it cannot be measured, it will not have any empirical data.
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