Simply put, dust is a small piece of everything around us.
For example, there are pieces of clothing. When we say "pieces of clothing," we are not talking about fabric fragments. It is a piece of thread that is used to make cloth. Most threads are made by twisting many very fine threads. Dust is the result of some of those fine threads becoming detached. As the term "cotton dust" suggests, cotton also produces a lot of dust, just like thread.
Another dusty substance is sand. Like cotton dust, there is a word for sand: "sandy dust. Sand in park sandboxes, for example, still has many large particles, and "sandballs" seem to be relatively rare. However, some sand crumbles and becomes so small that it is easily blown away by the wind. This is called a "sandy dust.
Tiny little pieces of various things float through the air on the wind. They are too small to be seen clearly by the human eye. However, even in the air we are breathing right now, there are tiny particles of dust.
This airborne dust may be very small and light, but it is still heavy. Therefore, it is attracted by the earth's gravitational force just like other things. That is the reason why the dust is floating around but will eventually fall down. Then, they accumulate as dust on top of the air and the glue.
In other words, dust exists wherever there is air. Therefore, dust will accumulate no matter how much you clean.