The unexpected result that the Universe is not expanding so slowly that it will clearly collapse back on itself in a Big Crunch, nor expanding so quickly that it will clearly keep expanding forever. Instead, measurements show that the Universe is treading a fine line between the two — the Universe is referred to as "flat". Astrophysicists would not expect this to be the case unless there were some cause for the Universe to tread such a fine line. Finding this cause is the "flatness problem".
Come, you lost Atoms, to your Centre draw,
And be the Eternal Mirror that you saw:
Rays that have wander'd into Darkness wide,
Return and back into your Sun subside.
From Farid al-Din Attar's
The Conference of the Birds