I can think of no discovery bigger in terms of correction of a belief if the metric is, How much bigger is something really than believed? Of course that is not the only significance. It is a major change in "world view" -- it seems like it would be equivalent to overturning helio-centrism (although perhaps astronomers having once had such a belief overturned would be more open to new ideas) and if so, it would have had impact far beyond astronomy itself.
There was no such discovery. Atomists believed in the plurality of worlds when mainstream astronomy was single world and geocentric. Although there is no a priori reason for the two to go together, historically the overturning of geocentrism went hand in hand with accepting the plurality of worlds. One reason was that to explain the absence of a visible stellar parallax (change in position of stars due to rotation of the Earth around the Sun) heliocentrists had to postulate that the stars are extremely distant, and then it was natural to assume that they were analogous to the Sun. In the ancient times the absence of parallax was one of the main arguments against heliocentrism, which was proposed as a hypothesis by Aristarchus and some others, and Brahe's chief objection to Copernicus was that the void between the last planetary sphere and the sphere of fixed stars would have to be implausibly huge to explain no parallax.
More philosophically, once heliocentrism turned the Earth from the center of the universe into one planet among many, and the metaphysical difference between Aristotle's sublunar and superlunar spheres was erased, the natural next step to take was that the Sun too was just one star among many. After that the "discovery" that Milky Way was just one galaxy among many was rather routine and anti-climactic, it is hard to even pinpoint when it "happened", see Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure.
So the closest thing to the "event" that the OP is looking for was the Copernican Revolution. Copernicus vaguely entertained the plurality of worlds already, although not publicly due to the cultural context of the time, a bit later Giordano Bruno was burned for it (along with other heresies). But with the spread of telescopes by mid 16th century the plurality of worlds was a commonality among scientists, Newton assumed it as a matter of course in Principia even though the stellar parallax was only detected by Bessel in 1838. Still, even the impact of the Copernican Revolution was enhanced by the cultural context that it displaced, as Renn explains in The History of Science and the Globalization of Knowledge:
"In response to the encompassing religious worldview, the new knowledge accumulated by these scientist-engineers began to assume the character of an equally all-embracing interpretation of the world, as can be found in the great philosophical concepts of the early-modern period, for instance, in the works of Giordano Bruno or Rene Descartes. Science eventually became a kind of counter ideology by which the emerging bourgeoisie could defend its claims to power, not according to a transcendent, religious order, but according to immanent laws of nature and society. The new knowledge thus also assumed a normative dimension.
This situation helps to explain why, in the sixteenth century, the reform of astronomy by Copernicus, placing the Sun rather than the Earth at the center of the universe, could have had such far-reaching ideological consequences: it occurred within a context of a socially dominant system of knowledge that claimed to be universal and exclusive. The geocentric worldview, placing the Earth at the center of the universe, was deeply anchored within this system of knowledge. Questioning this claim, even with good scientific reasons and without any intent of heretic provocation, still amounted to unhinging the whole system and thus causing an ideological revolution by means of an astronomical, and at the outset purely scientific innovation.
In contrast, there was no comparable revolution in seventeenth-century China when Jesuit missionaries introduced Copernican theory, or even Galileo’s telescope, which made the new view of the heavens so intuitively plausible. In Ming China, there was simply no combined religious and philosophical worldview that this new discovery could potentially provoke.