Sabine Hossenfelder explains succinctly and clearly at her Backreaction blog why a new paper proposing a negative mass solution to explain dark matter and dark energy doesn't work.
The most basic problem is that in Einstein's theory of General Relativity and pretty much any reasonable generalization of it, negative matter attracts negative matter, positive matter attracts positive matter, and negative matter repeals positive matter. But, this paper makes negative matter repel other negative matter, and also engages in baroque contortions in an effort to deal with the non-conservation of mass-energy associated with dark energy.
Ultimately, the resulting solution is worse than the cosmological constant and dark matter of the Concordance model of cosmology a.k.a. lambdaCDM, which is itself flawed.