Sources: Twilight, Luna, Telescope, Galaxy Cluster.

Twilight Sparkle stared at the enchanting image of the spiral galaxy through the eyepiece of the ornate telescope pointing out into the clear night sky. She admired the majestic swirling body of billions of stars and glowing gas. The curved arms bending around the heart of light.

“That’s amazing,” she said. She walked over to Princess Luna and rubbed her head against the fellow princess’s neck. “Your night sky is really a thing of beauty.”

The two alicorns were in a high observatory tower in Canterlot Castle, where Luna had set up a telescope to survey the heavens. The roof was open allowing a view of the cloudless summer night, perfect for stargazing. Around them the walls were lined with bookshelves housing a collection of astronomical treatises going back many centuries.

“Thank you Twilight,” replied the princess of the night. “But I cannot take credit for that galaxy. Raising the moon is one thing, but setting a trillion supermassive stars in orbital motion about their centre of mass is quite another. And besides—it’s over four million light years away. The light you are now seeing left its origin back when our equus simplicidens ancestors were still roaming the Pliocene prairies.”

Twilight chuckled. “Not even Starswirl the Bearded could do a time spell like that.”

“And yet,” said Luna, “Starswirl did have something to say on spiral galaxies. Appropriate enough given his name.” She levitated an ancient book off a high shelf. Twilight’s eyes lit up when she saw the name on the spine.

“Starswirl studied the motion of such galaxies, and the process by which they formed from the collapse of a large gas cloud.” Luna flipped through the pages of the book, pausing at a diagram showing the morphological classification of galaxies. “In doing so he uncovered the earliest evidence for dark matter.”

“Dark matter,” repeated Twilight, recalling the definition she had read in the Astronomical Astronomer's Almanac to All Things Astronomy. “The mysterious unknown substance which makes up most of the mass of these galaxies...”

“He measured the speed with which the stars in the outermost reaches of the spiral arms were moving,” said Luna. “And showed that these were moving so fast, one would expect them to fly out into space. Something must be holding them in, in addition to the gravitational pull of the visible stars.”

“Dark Matter?”

Luna nodded. “The hypothesis is that the galaxy is surrounded by a massive invisible halo. Its gravitational pull holds the galaxy together.”

Twilight the outstanding student thought about this hypothesis.

“Does it have to be dark matter? Could there be some other force out there that we don’t know about?”

“That’s possible,” agreed Luna. “But dark matter is the better explanation as it explains so many other observations. We see the same thing when we look at groups—or clusters—of galaxies. Individual galaxies move so fast we would expect the cluster to fly apart—but something holds it together. Or when we try to explain the Universe as a whole. Without dark matter it would just be an expanding cloud of hot gas. Galaxies and stars would never have formed. It fits all scales of the Universe, and we also see its gravity bend the light from more distant galaxies. But, you are right, this is just a theory—it is conceivable that galaxies take a Pinkie Pie approach to gravity. Yet, given the consistency, dark matter seems to be the simpler explanation.”

Twilight thought about this. A stream of photons, emitted millions of years ago by stars in a distant galaxy cluster, collided with her retina.

“But what is it?”

“That,” replied Luna, “is a very good question. The answer is still a mystery.”