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. Source : BBC

But there is no general agreement about the possible existence of Negative Mass - or what the implications might be. See: Wikipedia

Some proponents of the idea suggest that a universe which contained substantial amounts of 'negative mass' particles might be able to account for the large discrepancies in descriptions of the standard model of the universe - which currently requires 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. in order to explain how the observed universe apparently behaves.

See: Introducing the Dirac-Milne universe Astronomy & Astrophysics, Vol. 537

Also see: Massplugin-autotooltip__plain plugin-autotooltip_bigMass

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