I read that the fate of the universe depends on the amount of matter, or mass in the universe. If theres more than a certain amount of mass, gravity will eventually predominate and the unverse will contract into a singularity. If theres not enough mass, it will continue to expand forever, eventually fading into nothingness.

So far the observations indicate that there is not enough mass to make the universe contract. But, according to einsteirn matter and energy are interchangeable, so if the universe cools as it expands, the energy decreases. So I was thinking, if most of the energy in the universe were converted into to matter, would it be enough? Have the propeller-heads taken this into account in their calculations? I have also heard this new term, "Dark Energy". There may be energies that we cannot see or detect.

you gotta remember that 'C' in E=MC^2 is huge.  that makes 'E' huge and 'M' pretty small. 

in order to get a significant ammount of matter, you need an enormous ammount of energy. 

i'm not saying that there's not enough energy in the universe.  that i don't know.  i'm just saying that there must be a whole lot of it to make that kind of difference.