I am quite skeptic about these. They seem to be the easiest way for the cosmologists to explain what they do not understand. They find a mystery - the stars in galaxies rotate with the same speed around its center no matter how far from the center and they explain this phenomenon with something even more mysterious - the dark matter. They define dark matter as something that you cannot see - but it is there and casts its gravity over ordinary matter. Pretty convenient, isn't. Then they realize that the universe is expanding and here we go - the easiest way is to explain it with something "dark" again. And because matter is already occupied, let's pick "energy" instead.