They are phrases used to avoid the fact that the standard cosmological models don't conform to the actual data.
Some would call that response cynical. Not me, but some would.
"Those who have the power of accurate observation are often called cynical by those who don't have it."---George Bernard Shaw
By the way, since they don't know what it is, how do they map it? I envision it kind of like those 15th century maps of the new world with dragons and mermaids drawn all over the place.
Because we can observe the red-shifts of distant galaxies. The red-shifts are non-linear as a function of distance. The expansion should be flat (universe is infinite) or slowing down (gravity is slowing the expansion, possibly causing a re-collapse). But instead the father away the galaxies are, not only are they not slowing down, they are moving *faster* (the 1st derivative of motion -- rate of change of position if increasing over time is increasing), and in fact they are even accelerating as they go (the 2nd derivative -- rate of change of velocity is also increasing).
It's totally bizarre. Nothing in current cosmological models can explain it. Gravity just doesn't work that way. So physicists call it 'dark' (meaning unknown).