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 the standard model of the universe - which currently requiresand in order to explain how the observed universe apparently behaves.
See: Introducing the Dirac-Milne universe Astronomy & Astrophysics, Vol. 537
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