Yes, as far as I know, astronomers think so. Those are exoplanets. Planets without any nearby stars.

Even in our own solar system we don't know whats out in our Kuiper Belt. Probably nothing too big or we'd be able to measure the gravity effect.

But in the Oort cloud, we could have multiple Pluto sized things out there and have no idea.

An exoplanet is just any planet outisde our own star system. You're thinking of "rogue planets".

One of the proposed dark matter explanations is "massive astrophysical compact halo objects" (MACHOs). Brown dwarfs are among some of the possible MACHO candidates.