Have you ever noticed a dirty edge on someone’s carpet? Most people would think that the carpet is not being vacuumed on the very edge but this isn’t always true. Your carpet is a big filter and traps a lot of dirt. Homeowners need to also know how to properly use the vacuum they have and clean it regularly as well. Go to https://appliancehunter.co.uk/carpet-cleaner-support/how-to-use-a-vax-carpet-cleaner/ for a comprehensive guide on how to use vacuum cleaners properly. The dark edge on the carpet is due to air movement under that wall. Air is moving under the wall and could be moving in either direction depending whether there is positive or negative pressure. The carpet fibers at the edge are catching the dirt and creates the dark edge.
When the houses are being built the sheet rock does not go all the way to the floor. The base board is raised about a half inch off the floor to allow the carpet to be pushed under the board. This looks a lot nicer but allows air to move under the wall. The only way to fix this is to remove the carpet and use spray foam under the base board. This will seal any openings that will allow air to move under the wall.