Because it's true. When I got my PhD in cosmology, this is what everyone kept saying, including my thesis advisor, who is an accomplished professor in the field

Because it's true. When I got my PhD in cosmology, this is what everyone kept saying, including my thesis advisor, who is an accomplished professor in the field. So it's not just the author of this article. Gravity works a bit different at the scales we are talking about, but generally we would expect an expanding universe filled with only matter and radiation to slow down and eventually collapse back into itself. Or at least it would slow down and approach some constant velocity. Not unlike a ball that you kick into the air with below or above the escape velocity. But what we actually observed in the late 90ies is that is indeed not slowing down, but accelerating, as if some force was pushing it apart. The cause of this - whatever it may be - has been dubbed "dark energy". At these scales we don't think of gravity as a "force" like we did in Newtonian physics, so I don't think it's wrong at all to say that dark energy has the opposite effect of matter and is pushing the universe apart, i.e. has an repelling effect.