The Forerunners were the second known species to obtain sapience in this universe, after the long extinct Precursors. The species was known to study the remains of Precursor society, learning what they could. Much like the Precursors, the Forerunners vanished as well, having gone extinct, much like the Precursors.
Having existed during "Fold 2", some Ancients believe that it is still possible for Forerunner relics to still exist somewhere in the universe, though they would have been severely degraded with time at this point. Some more adventurous members of the Vacotor Knowledge Symposium actively seek these potential ruins and relics, but most consider it, in terms of percentage, far too difficult to locate for far too little information, especially since many state that it is unlikely that any of their technology is still in working order.
However, since Sal'jaedon and Vuul'huukos are "Fold 0" individuals, they both likely know a bit about the Forerunners. Unfortunately, neither has ever come forward with information on them; it does not help that both individuals are extremely hard to locate and speak to without first proving one's worth, which is extremely hard to accomplish.