When Henri Becquerel spotted that photographic plates became fogged if kept in a drawer next to uranium salts, the discovery of radioactivity was immediate.
By contrast, other scientific findings -- global warming, for instance -- take place incrementally, the result of gradually accumulating evidence.
Last week, scientists announced a small but potentially significant step in our slowly evolving understanding of what the universe is made of.
Astronomers have long known the stars contain the same atoms as those found on Earth.
But for years evidence has been growing that most stuff in the cosmos is not made of atoms or subatomic particles at all, but Something Else.
The first inkling that the universe is dominated by unseen material came from the observations of clusters of galaxies made in the 1930s. The astronomer Fritz Zwicky noticed that within the clusters, galaxies mill around so fast that the clusters ought to fly apart.
So what keeps them intact? The simplest explanation is that some form of dark matter provides the necessary gravitationally binding.
Today, cosmologists can put a precise figure on the amount of dark matter in the universe: about five times the mass of the luminous, common-or-garden variety of matter.
And its role in shaping the cosmos is crucial.
After the big bang that created the universe 13.7 billion years ago, matter was spread smoothly through space.
Aided by the gravitating power of the dark component, ordinary matter was pulled into clumps, which later evolved into galaxies that spawned stars, planets and, in one case at least, life.
A consensus has emerged that dark matter mostly consists of massive particles coughed out of the big bang. The reason for the appellation “dark” is because, unlike atomic particles, they have no electric charge, so cannot emit or scatter light.
Nor do they feel the strong nuclear force that traps protons and neutrons in atomic nuclei.
As a result, the dark particles interact so feebly with ordinary matter that they mostly pass right through it.
The race to detect and identify these particles started in the 1980s.
Because the solar system orbits the galaxy at more than 200 km per second, it should be ploughing through an ocean of primordial dark matter.
As a result, there is a small probability that a dark matter particle will bump into an atomic nucleus and send it flying.
The challenge is to detect such a collision.
One such experiment, known as the Cryogenic Dark Matter Search, in Minnesota, has been gathering data for several years.
Now, following painstaking analysis, project scientists have declared that they have recorded a couple of likely looking dark matter events. Theoretical physicists long ago predicted the existence of various weekly interacting massive particles.
One of these theories, called supersymmetry, links the nature of fundamental particles to the structure of space and time, and is an essential ingredient of string theory, the scheme that seeks to unify all nature.
Part of the rationale for building the Large Hadron Collider at CERN in Switzerland was to create what could be the very same particles that the Minnesota experiment may have detected coming from space.
Meanwhile, many other groups are planning experiments to elucidate the nature of dark matter.
Though it is too soon to open the champagne, if the Minnesota results are confirmed, they will represent nothing less than a transformation in our understanding of how the physical universe is put together.
Paul Davies is a physicist, writer and broadcaster, and director of the Beyond Center for Fundamental Concepts in Science at Arizona State University. His latest book is The Goldilocks Enigma: Why Is the Universe Just Right for Life?
— © Guardian Newspapers Limited, 2009