[yesthatjallen]

If you go back to high school physics class, you may remember that the more mass something has, the greater its gravitational pull. If galaxies were only made up of the stuff we can see, there wouldn't be enough gravity to keep them together (scientists reckon), much less to keep the stars in the sparse outer edges orbiting just as fast as those in the center (scientists reckon). In fact, scientists reckon that normal matter makes up less than five percent of the universe. Dark matter seems to make up a whopping 27 percent. (The rest is a mysterious force called dark energy.)

"Scientist reckon".

Maybe scientists reckon wrong.

[LibWhacker]

there wouldn't be enough gravity to keep them together (scientists reckon)

LOL, your use of 'reckon.' But I reckon you reckoned wrong. Ditto your other use of 'reckon.' They did much more than "reckon" how much gravity it would take to keep the stars together, and the outer stars orbiting just as fast as the stars in the central region of the galaxy.

[Redcitizen]

(The rest is a mysterious force called dark energy.)

So that’s the new phlogiston.