Even the mighty Galactus, the terrible Devourer of Worlds, is scared of something, or rather of someone: the Molecule Man, who could just be Marvel's most powerful villain. This absolutely crazy character has the power and authority not only to summon Galactus on a whim but also to destroy the Devourer with a mere thought.
Galactus is the most famous of Marvel's cosmic beings, the last survivor of a previous universe who feels the compulsion to feed on the energies of entire planters. While he was introduced as a "regular" villain for the Fantastic Four, Galactus is in fact a universal force who occupies a specific place in the multiversal hierarchy. However, despite all his mastery of the Power Cosmic, there are creatures who stand above Galactus in that same hierarchy, or outside of it. One of these outsiders is Owen Reece, aka the Molecule Man, who is apparently a supervillain who acquired his powers of matter manipulation (and lost his sanity in the process) through an incident with a Cosmic Cube but is in fact the embodiment of a much more powerful and sinister force.
Both Galactus and Molecule Man went through significant changes during their histories. For a period of time, Galactus was transformed into the Lifebringer, whose purpose was to give life to the universe rather than destroy it. In that form, Galactus entered into a conflict with other cosmic beings, Lord Chaos and Master Order, who resented his transformation. In The Ultimates #6, by Al Ewing and Christian Ward, the Lifebringer wins that battle easily, but he is then immediately summoned by someone he actually fears, the Molecule Man, who has become a sort of cosmic demiurge after the events of Secret Wars. Answering the call, Galactus outright admits to being scared of Reece, because he could end his existence with a simple thought.
Even Galactus Is Afraid Of The Molecule Man (And He's Right)
It's really unusual for Galactus, who is the personification of the Power Cosmic, to be afraid of something, and yet he has good reasons to be. The incident that gave Reece his powers was actually caused by the Beyonders, omnipotent beings who exist beyond the Multiverse. They created the Molecule Man as a singularity, the same in every reality, to function as a bomb that would destroy its universe once triggered. During Secret Wars, Reece became the conduit for the infinite power of the Beyonders, who were apparently killed by Doom. When the Multiverse was recreated as the Eighth Cosmos, the Molecule Man further evolved, becoming capable of editing and ordering the basic information of reality, and calling himself "Information Man" or "Narrative Man", as proof that his status goes beyond that of regular Marvel characters (who are "trapped" in the narrative), including Galactus himself.
Owen Reece may not be the most famous of Marvel's cosmic beings, but his power goes way beyond theirs, including Galactus and even the Living Tribunal. The Molecule Man has only made sporadic appearances since his meeting with Galactus, but one day, if his mental state becomes unstable again, he could become the most dangerous villain that the Marvel Universe ever had to face.