explanation for dark energy is that it is a property of
space. Albert Einstein was the first person to realize that
empty space is not nothing. Space has amazing properties,
many of which are just beginning to be understood. The first
property that Einstein discovered is that it is possible for
more space to come into existence.
Another explanation for dark
energy is that it is a new kind of dynamical energy fluid or
field, something that fills all of space but something whose
effect on the expansion of the Universe is the opposite of
that of matter and normal energy call "quintessence".
Cosmism suggest that here
is a possibility is that Einstein's theory of gravity is not
correct for very long distances, like it is doesn't affect
elementary particles on very small distances. That would
only affect the expansion of the Universe, but it would not
affect the way that normal matter in galaxies and clusters
of galaxies behaved.