I'd postulate that they weren't!

This answer by u/BRIStoneman goes into their record in battles - apart some from high-profile near-successes that get talked about a lot, the Norse raiders lost more often than they won.

But, I'd like to go a bit farther and take a look at where they "conquered", and suggest that the framing of military might is not necessarily helpful.

The British Isles

This is kind of the one place where a "conquest" narrative works out really well - the founding of the kingdom of Orkney involved military defeat and displacement of the (primarily Scottish) peoples living there prior to the Viking Age. Additionally, the 9th century "Great Heathen Army" did disrupt elite political structures in Northumbria, Mercia, and East Anglia, setting up kings who at least allied with them, if they weren't Scandinavian themselves.

In Ireland, you get the kingdom of Dublin, which is certainly not nothing, but it was immediately embroiled in 2 centuries of being another player in Irish politics, alongside all the other petty kingdoms. Despite the violence and the cultural memory of the raids, on the ground it doesn't appear that they actually turned into political on cultural dominance.

North Atlantic

Here's where a significant part of the "territory" of the Viking Age comes from. But, for the Shetlands, Faroes, and Iceland, there was at most Irish ascetic hermits living on the islands. Iceland may well have been completely deserted by the time Norse settlement picks up in the 870s. While this adds numbers to the map, framing it as a "conquest" implies that there was conflict and defeat of an organized state, which doesn't make sense for the entire North Atlantic.

Normandy

Normandy is an interesting case, because the Royal Frankish Annals and the Annals of St. Bertin make it fairly clear that there was significant Viking activity in Normandy. However, archaeological evidence that settlements prior to the granting of the title of Normandy to Rollo did not last very long! Additionally, that granting, even if it is a form of victory, still placed Rollo in a position of direct obligation to Charles the Bald, and culturally, the Viking "rulers" acculturated very thoroughly to French culture within a century. Certainly, the raiding presence that caused this eventual "hiring" took advantage of severe military weakness and instability within Carolingian West Francia, but a narrative of conquest and displacement, so much as disruption are

Rus'

This is another that is very weird. The Primary Chronicle says that warring Slavic groups invited a group of Scandinavians to rule over them and solve their conflicts! But, archaeological evidence fills it in a little bit more: from before the Viking Age, both violent and trading expeditions were very frequent to the Baltic coast and Slavic areas. These traders formed emporia, or trade towns, along the rivers, which turned into large communities. How exactly they became the elite of the Slavic-majority state of the Rus' up until the mid-10th century is somewhat mysterious, because the Primary Chronicle is from centuries later, but if it is peaceful interaction that led to local chieftains inviting them in, I would argue that should not be seen as conquest!

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Certainly, there is room for multiple frameworks here - I do not claim that "conquest" is a meaningless term for this period, nor that violence did not contribute to Norse success outside of Scandinavia - the frequency and brutality of raids is widely accepted. However, I follow recent scholarship here in the re-framing the Norse expansion as a diaspora instead of an invasion - doing so asks "what does calling it a "conquest" obscure about the networks of interactions present across Europe in the Viking Age"? The answer is, of course, a lot - the Vikings interacted and influenced the early middle ages in many ways, and violence was only one of them.