The Universe appears to have a value of $Omega$ very close to $1$. This means the internal angles of a triangle add to $180^@$ which is very odd, because it is far more likely that there will be an excess of mass-energy in the Universe (meaning $Omega$ would be greater than $1$ and the internal angles $<180^@$) or too low an energy density, meaning $Omega <1$ and the internal angles $>180^@$.
It's just weird that it is so very close to $1$, with no a priori reason for it having this value. Cosmologists, feeling honest, accept it's a bit embarrassing.
The Wikipedia article is excellent.
