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Class G - Uncontrolled Airspace
Believe it or not, there are still parts of the modern world where one can fly IFR without the benefit of air traffic control services. These "nether-regions" fall into the category known as uncontrolled or Class G airspace.
ATC is specifically mandated to not accept responsibility for separating aircraft operating in Class G airspace. In such areas, pilots are not required to request altitude or route changes, they can do so at will. As well, the pilot is responsible for all terrain clearance. In mountainous terrain such as the Rocky Mountains and Alaska, pilots must be extra vigilant when flying in cloud without the benefit of the controller's watchful eye.
Recently, I observed two aircraft in uncontrolled airspace, heading nose to nose and at the same altitude. With about 30 miles between them, I issued a traffic advisory to the one aircraft as the other was not in contact with ATC, nor were they required to be. It was not my responsibility to separate those aircraft, but a traffic advisory was issued and it may have prevented a collision.
If one of the aircraft had been below radar coverage, no advisory would have been issued. Remember to leave your transponder on when flying in uncontrolled airspace. Often a controller will be able to see the target on radar and issue an advisory. If you are out of radar coverage, the TCAS on many aircraft needs a will still pick up functioning transponders.
All things being equal, the benefit of an extra set of eyes will always provide a greater margin of safety.