I wanted to know that what is the most basic or the basicestest thing that makes up the universe? Just like a wall is made up of bricks and bricks are made up of atoms and atoms are made up of subatomic particles, what would be the last thing we would encounter? Is it a mass or a massless thing or a phenomenon or what?
If you wish to get an answer, the only answer I can give is, the "Consciousness".
From your question I guess that your question is, "Where will we stop when we explore the root of everything?".
You know that when we go deeper into a thing we get things that cannot stay in one particular atom as you imagine, isn't it? Its subtlety becomes unimaginable. What I said is just for guiding your imagination; not for any kind of comparison.
Since you can never go beyond it, "Consciousness" is the basic of all things...if you wish to get ONE such thing. See: https://en.wikipedia.org/wiki/Chaitanya_(consciousness). You would get the answer to your last 2 questions from this link.