The Enigma Force is a mystical energy field that permeates the Microverse, a subatomic universe. As the name implies, no one truly understands it. In a sense, it is the Microverse, and its first priority is survival. It does not (and perhaps cannot) act directly; rather, it works through the Time Travelers to protect and preserve the Microverse.
One million years ago, the Wanderers came to Earth from many worlds. There they faced extinction at the hands of spectral beings called Whirldemons. With a magical blade called the Sword in the Star, Prince Wayfinder created a new universe for the Wanderers and imprisoned the demons. Wayfinder became the first Time Traveler.
One thousand years ago, Commander Arcturus Rann set out on a voyage to explore the Microverse. While his conscious mind spent a thousand years in suspended animation, his subconscious made contact with the Enigma Force. At intervals, astral copies of Rann would appear and become new Time Travelers. Some of them became Shadow Priests, who set up churches devoted to worship of Dallan and Sepsis (Rann's royal parents). All this activity prepared Rann to confront Baron Karza when he returned home. At the proper moment, the Shadow Priests shed their disguises and reunited with Rann, giving him the power to defeat Karza.
The Enigma Force came into play again when the Spacewall between the Microverse and the Macroverse (i.e. the Marvel Universe) began to break down and the Whirldemons escaped. Rann had lost his connection to the Enigma Force, which meant that many of the Time Travelers ceased to exist (or were inaccessible). While Rann tried to restore that connection, the Microns (with the help of Doctor Strange) went on a quest to reclaim the Sword in the Star, drive back the Whirldemons, and repair the Spacewall.
Recently, the King of the Whirldemons escaped his prison on Earth and attempted to reopen the Spacewall. To counter this, the Enigma Force gathered three of its Macroversal former hosts of the Uni-Power (see "The Uni-Power" below): Spider-Man, the Invisible Woman, and X-23. The Enigma Force recognized its special bond with X-23, the latter having chosen its power to escape hell sometime earlier. X-23 eventually defeated the King of the Whirldemons by allowing herself to be possessed so that she could return it to its prison.
Before it left her, the Enigma Force informed Laura that she is the future heir to its power, telling her that she was its chosen vessel and that the preceding events were only the beginning.
During the Karza Wars, a Time Traveler first bestowed the Uni-Power on someone in the Macroverse. The Uni-Power is not the Enigma Force itself but only one facet of it. It allows anyone anywhere to become Captain Universe in times of great danger. Like the Enigma Force, the Uni-Power is sentient to the extent that it will not willingly allow itself to be used for evil.
Recently, the Uni-Power itself began to fluctuate. It merged with several superbeings to find the source of its problem.
Time Travelers typically appear as glowing green ethereal figures that strongly resemble Commander Rann. They are almost as mysterious as the force they serve. They observe, but they do not interfere. They hold great power, but they give it to others to use. The Shadow Priests claimed that they were only shadows next to the light of the Enigma Force.
Obviously, the Time Travelers can move forward and backward in time. They see possible futures, and when they detect a threat to the Microverse, they take steps to counter that threat. In their Temple of Time, they sift through clues and formulate strategies. They are not omniscient; often, their plans do not bear fruit until the last minute, when the fate of the universe may hang in the balance.
They are limited by the abilities of their agents. In Rann's case, when he lost the ability to tap into his subconscious, the Time Travelers had no way to communicate with him, much less manifest their power in him.
As individual entities, they are vulnerable. Karza was able to capture several and drain energy from them. They are also able to act independently of the Enigma Force and travel to other universes.