This depends on the total amount of mass in the Universe. Many cosomologists believe there is good reason to expect that we should be above critical mass, that is, that the UNiverse will end up contracting (why they believe this is above my level of understanding). Though we can't observe all the matter for this to happen. We do know, though, for instance, that much of the matter in galaxies is not directly observable, we know its there from its affects on the observable matter in the galaxies, we can't detect it otherwise. Hence "dark matter". The nature of the dark matter is not known for sure, there are a few viable theories. Compact objects, such as white dwarfs, brown dwarfs, and neutron stars, or black holes even, are candidates, as they could be huge amounts of these objects that we cannot see simply because they emit very little radiation. Current efforts to detect these objects in our own galaxy through gravitational lensing is underway.
Of course, it could be that there isn't enough matter to cause a slow down, which would mean the universe would expand forever, and eventually become nothing but a vast expanse of cold iron.
Eventually, provided the universe does not contract and collapse on itself before this happens, yes, all the matter in the universe will be converted to the most stable element, iron. However, this is a long long way off. Our own sun is about 70% hydrogen 25% helium. About 5 billion years from now, it will die, but still it will be mostly hydrogen, as only the matter in the core is hot enough to fuse.
How is that a guess?
The laws of the Universe only apply to the Universe, how could they apply elsewhere?
