  Dark sector, constituting about 95 percent of the Universe, remains the
subject of numerous studies. There are lots of models dealing with the cause of
the effects assigned to dark matter and dark energy. This brief review is
devoted to the very recent theoretical advances in these areas: only to the
advances achieved in the last few years. For example, in section devoted to
particle dark matter we overview recent publications on sterile neutrinos,
self-interacting dark matter, dibarions, dark matter from primordial bubbles,
primordial black holes as dark matter, axions escaping from neutron stars, and
dark and usual matter interacting via the fifth dimension. We also overview the
second flavor of hydrogen atoms: their existence was proven by analyzing atomic
experiments and is also evidenced by the latest astrophysical observations of
the 21 cm spectral line from the early Universe. While discussing non-particle
models of the cause of dark matter effects, we refer to modified Newtonian
dynamics and modifications of the strong equivalence principles. We also
consider exotic compact objects, primordial black holes, and retardation
effects. Finally, we review recent studies on the cause of dark energy effects.
Specifically, we cover two disputes that arose in 2019 and 2020 on whether the
observations of supernovas, previously interpreted as the proof of the
existence of dark energy, could have alternative explanations. Besides, we note
a study of 2021, where dark energy is substituted by a new hypothetical type of
dark matter having a magnetic-type interaction. We also refer to the recent
model of a system of nonrelativistic neutral gravitating particles providing an
alternative explanation of the entire dynamics of the Universe expansion.
without introducing dark energy or new gravitational degrees of freedom.
