so, stars work on fusion, until they are unable to use heavier elements (iorn) in their reactions, and then they die. when stars have turned all matter into iron, what next? iron is still matter, will it not be attracted to more iron and create some even bigger stars that are so giant with huge forces they can use iron in their reactions, or will the "iron stars" turn into black holes and eat all the matter?, this is assuming the universe wont implode on itself but will keep going and going and going and going....
Problem is that you need to add energy to get iron to fuse. Up to iron all fusions produce energy.
What happens when stars die is that they basically have a chunk of iron in their core. This does not fuse as it needs energy so the outward pressure from nuclear fusion stops. This however means that there is nothing opposing gravity so gravity pushes inward which in some cases can lead to a nova in which the gravitational energy crushing in on the iron provides the energy needed to get it to fuse into heavier elements.
Once iron is fusing there is no outward pressure so what happens will be happen quickly as gravity implodes the star. What you are left with depends on how big the star was to start with, black holes, neutron stars etc.
You wont really every end up with a big block of iron floating around in space that used to be a star. (Though it might be theoretically possible if the stars atmosphere was accreted off to say a binary pulsar which kept gravity from collapsing too far - very long shot that).
Whatever you are left with bill probably just stay floating around in space and perhaps occasionally draw in more matter. Collisions are probably rare. There does not seem to be any evidence of large scale collisions even in globular clusters which we think of as being 'denley packed' with stars. Similarly the Magellanic clouds pass through the milky way with trails of gas left in their wake but little evidence of direct collisions.
  