The string, according to the most current theories. I'm not real clear on what a string actually is, but I don't think anyone else does either. Not even the physicists who have been studying it for thirty years really claim to understand string theory.

The Quark-the smallest unit of matter Three quarks make up a proton or a neutron The proton seems to be made of two up quarks and a down quark The neutron seems to be made of one up quark and two down quarks Accelerator experiments in the early 1970s began to provide experimental evidence supporting their existence The quark model is the best theory we have today to describe the structure of atoms and molecules Quarks are one type of matter particle. Most of the matter we see around us is made from protons and neutrons, which are composed of quarks. There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.) Be glad that quarks have such silly names -- it makes them easier to remember! Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge, which we will discuss later. The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years. Want to see a particle physicist's idea of a good pun?

There isn't a smallest unit. Contrary to the car audio expert, the quark is not the particle with least mass. Nor is it the electron, whose mass is at least three times smaller than the lightest quark. Instead, look to the neutrino. It is known that neutrinos have mass, but the mass is so tiny we don't know what it is!

the disnae matter :-)

Is this dark matter, dark dark matter, or is it about some other matter? Does it really matter?

The dusbar. Not yet realized.

The trivion.

Tachyons are probably the smallest unit of matter since they theoretically have imaginary mass and travel faster than the speed of light.

I am in eighth grade and all it says in the book is an atom. Thank you all for allowing me to amaze my teacher when i put down quark for the answer! (My dad is a Nuclear Physcian so he explains lots of physics to me.) Thank You!