When Fluttershy was pushed from Cloudsdale before becoming a Harmony element bearer, she was unable to slow her descent due to her atrophied wings. Indeed, it is not unreasonable to assume that she reached terminal velocity during her fall, which should by all accounts have resulted in an interesting yellow impact crater directly below the cloud city and at least one murder charge. Instead, she was saved by a group of passing butterflies- not only an extremely unlikely and amazing event, but one that could not be possible unless the butterflies were made of a as-of-yet undiscovered super-dense substance. Ergo, dark matter. QED.