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".
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass