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The entire universe is made of two things: matter and energy. For the time being we will concern ourselves with matter only.
Matter is defined as that which has mass and occupies space. This definition seems simple enough, yet there are profound issues which surround it. For example, there is currently no generally accepted scientific theory for why mass exists. It can be defined:
mass characterizes an object's resistance to a change in its motion.
But why it exists cannot yet be demonstrated. Mass just is, it just exists.
As we look into it more, definitions start to become circular, as in the definition of space:
space is that which is occupied by matter.
So, any definition of space exists only if there is matter. Take the matter away and space ceases to exist. Of course, saying "take the matter away" is very easy to say. It is quite another thing to specify what is meant by it.
As fun as these philosophical issues are, it's time to move on!!
All matter is particulate in nature. This basically means that between separate bits of matter there are spaces which contain no matter. In science it is called the "atomic nature of matter." It is generally agreed that the Greek philosopher Leucippus and his student Democritus were the first to propose this idea, about 440 B.C.. This "atomic theory" (to use a modern phrase) was opposed by Aristotle 100 years later, who taught that all space is filled with matter, that there are no empty spaces. Aristotle's ideas were accepted as correct by almost all educated people, until the early 1800's, when atomic ideas began to be more generally accepted as correct. There is a tutorial on this topic on this web site, titled The Greek Concept of Atomos: The Indivisible Atom.
Today, we know that there are many different particles which make up matter. Some are long-lasting, such as the proton, while others are very, very short-lived, such as the top quark. The primary "particle" in chemistry is the atom. However, you probably know that there is a substructure to an atom; that it is made of protons, neutrons and electrons. You may also know that protons and neutrons are each made of three quarks. There are many other particles beyond the proton and neutron, some containing two quarks and some containing three.
There are two other categories of particles which appear to NOT be made of quarks: electrons and neutrinos. As far as science is currently able to tell, there are three types of particles with no substructure that we can detect: quarks, electrons and neutrinos. It may be that someday we will learn the the electron, for example, is made of still smaller pieces like an atom is made of protons, neutrons and electrons. That would be pretty cool!!
There is also a fairly sophisticated concept called "virtual particles." While it is based on some concepts you have not yet learned, it is still fairly easy to describe in a general way. There is energy in the universe in addition to matter, we are just ignoring it for the time being in this tutorial. Some of the energy can spontaneously merge to form a particle of matter. (Einstein showed that matter and energy can be converted, one into another.) These "virtual particles" exist for very, very small amounts of time, so small that we cannot detect them before that pop back into being the energy they were made from. (These is another concept, called the Heisenberg Uncertainity Principle which affects the lifetime of virtual particles, but I will not go into here.)
Since the universe is flooded with energy, this means that, in every square centimeter of the universe, are trillions and trillions and trillions of virtual particles popping into existence every fraction of a second and then going back to energy. "Empty" space is actually a boiling cauldron of particles, popping into and out of existence. Even though individual particles cannot be detected, their effects can. If you want to look into this further, look up the "Casimir Effect" as well as learn what "polarizability of the vacuum" or "quantum fluctuation of the vacuum" mean. It may be that Aristotle was right all along.
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