The fundamental particles that frequent and shape our world come in a dizzying variety. There are particles that whiz by unannounced and particles that seem to linger forever. One is as heavy as an atom of gold; another weighs nothing at all. Some congregate in small groups; others prefer to go it alone.

In the last few decades, physicists have developed a theory that makes sense of this apparent disarray. The Standard Model, as their theory is called, is a triumph of modern science.

Not only does it reduce to a handful the number of basic particles composing matter and explain how they are related. It also tells us what these particles can do and what they can't, how they come together and how they fall apart — all at dimensions a billionth of a billionth of the human scale.

Physicists have repeatedly challenged the theory with experiment, but the behaviors the theory described always materialized. Measurements matched theory decimal for decimal.

The Standard Model even described particles and forces that no one had ever seen before, but later found — a stunning confirmation of the theory's enormous predictive power.

Like Mendeleev's periodic table of the chemical elements, the Standard Model has simplified the physical world. But now physicists are looking for the equivalent of an atomic theory to explain its whys and hows.

Intent on answers, physicists have pressed ahead with mathematical theories to extend the Standard Model.

Hiding behind the quarks and gluons, the equations say, lie new particles and new forces — indeed, a whole new physics.

With powerful tools coming on line to pry deep into both inner space and outer space, physicists will soon begin to tell us why matter is the way it is.