When physicists and astronomers concentrate on the normal atoms and energy that make up the well-understood stuff of our universe, our stars and planets and us, they're only dealing with 4 percent of the total. (And when we point telescopes into the sky, the part of this normal matter that glows in optical wavelengths amounts to a scant 0.5 percent.) A full 96 percent of all the stuff that is out there goes largely undetected and is far from understood.
I don't know whether to be amused or alarmed.
It turns out dark matter amounts to 21 percent of the total mass-energy of the universe, and dark energy adds up to a full 75 percent. (The use of the adjective "dark" in both names is misleading. They have nothing to do with each other. Dark matter pulls in like normal matter does, helping interstellar gases and dust clump together to form stars, galaxies and galactic clusters; dark energy pushes out, accelerating the expansion of space itself.)
Scientists think dark matter is mostly what they call WIMPS - weakly interacting massive particles - that are strange particles indeed. Look in vain for such normal components of matter as protons and neutrons. It's a bit of an embarrassment that years of elaborate experiments have yet to detect their presence. Still, the dark matter exists. We wouldn't be here without it.
Dark energy, making up three-fourths of the mass-energy of the universe, really isn't understood at all. But it is there, and four or five billion years ago its outward-pushing power overcame that of gravity and stepped on the gas. There are theories about it - having to do with quantum fluctuations and something called quintessence - but I'll pass on the explanations. This is heavy-duty physics that wouldn't fit in a blog, even if a former small-town journalist could do it justice. Let's just say it is energy with negative pressure that speeds up the expansion of the universe. And the foot may stay on the accelerator forever.
I've had a layman's fascination with astronomy all my life. I've read many, many books of popular science, and I've just been watching a long series of up-to-date lectures on the subject. Physics and astronomy have an amazing record of accomplishment. On the other hand, apparently it's 4 percent down, 96 percent to go.