All physical objects have ‘mass’. The mass is (largely) a consequence of the additions of the masses of the atomic and subatomic particles from which the objects are comprised. Theorists currently support the idea of mass-creation via the Higgs Mechanism. See Wikipedia

But Higgs theory doesn't explain how or why subatomic particles themselves have wildly differing masses. For example, the 'up' and 'down' quarks which make up protons and neutrons, have different masses (by a factor of 2) - and the 'top' quark has a measured mass approaching that of some entire atoms.

There is also no explanation as to why the masses of some particles are "100 quadrillion times smaller" than theory would suggest. Source :'The Higgs mass mystery' New Scientist, Nov. 2015.

Further, as well as the current theories that put 'ordinary' matter as accounting for just 5% (or so) of the mass of the universe (see: Dark Matterplugin-autotooltip__plain plugin-autotooltip_bigDark Matter

"The nature of the dominant component of galaxies and clusters remains unknown."

Source : Measuring the dark matter equation of state (Mon. Not. R. Astron. Soc. 415, L74–L77)"

In the 1930s, astronomical observations of galaxy rotations showed that the outer regions were rotating (about the galaxy's 'centre') at the same speed, or faster, than the central regions. Subsequent calculations referring to the galaxy's mass, and thus its internal gravitational attractions, showed that i… and Dark energyplugin-autotooltip__plain plugin-autotooltip_bigDark Energy

"The observational results of the latest 15 to 20 years have established a standard model for the cosmology which has some amazing consequences. A mysterious entity, the dark energy, has been confirmed as the dominant component of the Universe, and is also responsible for its accelerated expansion.) the Higgs theory only accounts for around 2% of this 5%.

[…] these current-quark masses leave 98% of the mass of the atom unexplained."

Source : Physik, Volume 526, Issue 5-6 Pages 215-226

A further complication is that the mass of the Higgs particle itself has yet to be explained. "[…] the problem cannot even be formulated in the strict context of the Standard Model"

Note: A recent (2016) (US/Japan/China) study details theoretical and experimental research on super-cooled rubidium which claims to demonstrate 'Negative Mass'.

Theoretically, a 'Negative Mass' object would move towards a force attempting to accelerate it, or would fall upwards in a gravitational field (etc etc).

Negative mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative mass."

Source : Negative mass hydrodynamics in a spin-orbit–coupled Bose-Einstein condensate
Phys. Rev. Lett. 118, 155301

See a separate article on this concept : Negative Massplugin-autotooltip__plain plugin-autotooltip_bigNegative Mass

The idea that some exotic forms of matter might have 'negative mass' was first proposed in the 1950's.

In 2017, a team from Washington State University described an set of experiments which claimed observation of negative mass particles.

Importance Rating