At the outset, it is important to point out that we have no solid documentation to address your question from the period. We can only surmise based on what was documented in Scandinavia some thousand years later. Projecting backwards like this is always risky because folklore like all culture changes with time.

The other issue is the word "average": identifying a typical or average set of beliefs is problematic because without dogmatic texts, folk belief tends to wander a great deal, with variation from person to person, place to place, and generation to generation. Using the problematic approach of looking back based on much later documented belief systems, it is fair to say that non-believers and skeptics are ubiquitous even while others believed, and we can conclude that the same was true in ninth-century Norway.

Also projecting backwards, we can consider what Norwegians believed a thousand (or more) years later and then to consider what that might imply about previous, undocumented generations. Fortunately, there has been a great deal of work in this regard, so we have a place to start. Elisabeth Hofelich-Hartmann (1912-2005) published her doctorial thesis, Die Trollvorstellungen in den Sagen und Märchen der Skandinavischen Völker – The Troll Beliefs in the Legends and Folktales of the Scandinavian Folk - in 1936. It was written under the direction of two key, important Scandinavian folklorists: Carl Wilhelm von Sydow (1878-1952) and Sven S. Liljeblad (1899-2000). For a treatment of Hartmann and her relationship with these two scholars, see my essay, Nazis, Trolls and the Grateful Dead.

Hartmann described folk beliefs that were still vivid and active into the early twentieth century, especially in rural areas. There is every reason to believe that what she documented and analyzed would have been very similar to the level of belief eleven hundred years earlier.

Northern Europeans describe their respective supernatural beings in legends, accounts generally told to be believed. These same supernatural beings can appear in folktales - narratives generally told as fiction, but just because they appear there as well does not mean that they were thought not to exist: folktales also include people and animals that were known to exist. There have also been what von Sydow labeled as "ficts" - things that one tells to children for one reason or another, including to warn them away from dangerous situations. Parents don't necessarily believe these things, but they employ them strategically - as occurs with stories about Santa Claus. Just because a supernatural might figure in a fict does not mean that there were not general belief in that entity; I merely bring it up to help with an understanding of this subtype of oral narrative - since you mentioned it.

In general, Scandinavians of the nineteenth century told legends about a variety of supernatural beings that were referred to with various terms including "troll" and what can be translated as "hidden folk" as well as a variety of local terms that refer to the sort of dwarves, elves, fairies (etc.) that are more familiar to English-speaking readers. We have some hint of what the counterparts might have been several centuries earlier thanks to Icelandic sagas and other sources, but these are imperfect because they are often not explicit, they are often from Iceland (not Norway), and considerable time elapsed between 800 and when the saga literature was being recorded. The possibility of cultural/folklore drift is a concern.

Nevertheless, all this evidence suggests that typical people in 800 in what is Norway today would be familiar with the beliefs in entities that were the antecedent of what was documented beginning a thousand years later. These people would have regarded them as likely real - and very dangerous. They would have taken precautions to avoid harm and especially abduction, which was the leading concern throughout Northern Europe when folklore was being documented beginning in the nineteenth century. This concern is so widespread, that it easy to imagine projecting back to our earlier period - even though that process is problematic.

Again using our flawed method, these entities were not vague negative energies, but rather they were very real, corporeal creatures. They were capable of vanishing, so they were not exactly like "normal" creatures, but they were able to be as tangible as people.

At the same time, Hartmann documented more vague forces that could occupy a place (although most of her discussion in this regard was based on Swedish traditions); the point here is that the folk understood the idea of nebulous forces in various locations, and they made a distinction, separating these things from the more tangible entities that were the focus of so much concern in their beliefs and legends.

I hope this helps!