If there is anything I have read numerous times, it's that physicists (and by association astronomers), love symmetry. And there's a lot of symmetry about in the natural world.

Of course not all of nature is symmetrical, such as the 2nd ‘law' of thermodynamics (time's arrow). However, that ‘law' is really only a prediction of possibilities. Therefore, it's not really a universal physical ‘law' at all (IMHO). If you wait long enough (under the guise that ‘everything not forbidden is compulsory') that ‘law' will be broken. For example, sir the cream into your black coffee, and sooner or later (probably much, much, much later) said cream mixed into said coffee will separate out to one side leaving, however briefly, black coffee on the other side.

Symmetry in the physical sciences has had a history of making successful predictions. For example, observed symmetry within the three families/generations of the fundamental particles enabled the prediction of the existence of some missing theoretical particles that would have to fall within in order to make the symmetry come out right. Naturally, these theoretical particles were found, spot on where symmetry predicted.

A more familiar example is the Periodic Table of the Elements. When the overall order or symmetry of the chemical elements was realized, it was further realized that there were gaps – undiscovered elements. Of course it came to pass that all of these predicted elements, required to exist in order to save the symmetry, were eventually discovered.

A third example was the prediction of the positron – antimatter's symmetrical  equivalent of the electron – way before it was the positron was actually detected. 

So, I'm going to play a little game of predicting things based on symmetry!

As a starting point, we all can agree that our Universe contains ordinary (baryonic) matter – electrons, protons (comprised of quarks), neutrons (composed of quarks again), etc.

There is some symmetry here, because it is also well known that there can exist ordinary (baryonic) antimatter – positrons instead of electrons; anti-quarks instead of quarks, etc. 

If that were that, all would be well. However, yet another form of matter has been discovered in our Universe that goes by the phrase ‘dark matter'. Astronomers have verified that ‘dark matter', apparently, must be non-baryonic in nature, and nobody really has a solid clue as to exactly what ‘dark matter' actually is, other than it's not electrons, protons, neutrons, positrons, anti-quarks, etc.

But that breaks our nice symmetry of baryonic matter and baryonic antimatter on one side, and non-baryonic ‘dark matter' on the other. To restore symmetry, we also should have non-baryonic ‘anti-dark matter' as well. That awaits discovery (if it exists) and thus I call that the first of our undiscovered forms of matter.

The one thing that both ordinary baryonic matter (and ordinary baryonic antimatter) have, as well as non-baryonic ‘dark matter' (and presumably its antimatter counterpart), is that they all have properties of positive gravitational attraction. A ‘dark matter' planet would orbit a ‘dark matter' star just like Earth orbits the Sun (assuming ‘dark matter' is the type of matter that can actually form macro objects like planets and stars).

Symmetry has been found to be preserved now when it come to gravity. Relatively recently (1990's), astronomers discovered a new form of energy which has been called ‘dark energy'* (although ‘dark matter' and ‘dark energy' probably have no real connection apart from the label ‘dark' – ‘dark' because we can't directly ‘see' either). That ‘dark energy' is not a pulling force like gravity, but a pushing force that's causing the expansion rate of our Universe to accelerate (when everyone expected good old straight forward gravity to slow the expansion rate down). Translated, ‘dark energy' is, for all practical purposes, antigravity.

But oops, ‘dark energy' apparently breaks symmetry with the matter component of our Universe. According to Einstein, energy and mass are but two sides of the same coin (E equals M C squared). A tiny bit of ordinary mass (or matter) gets converted to ordinary energy say when you burn a piece of paper; a greater percentage of ordinary mass (or matter) gets converted to ordinary energy in say, a nuclear explosion; and 100% of ‘ordinary' mass (or matter) gets converted to ordinary energy when ordinary mass (or matter) meets ordinary antimatter mass. And, the reverse is possible – M equals E divided by C squared).

Now I assume that ‘dark energy' is real energy seeing as how it has the power to accelerate the expansion of The Universe. I further assume that ‘dark energy' is as obeying of Einstein's equation as any other form of energy. So, what kind of mass (or matter) can be converted via Einstein's famous formula to ‘dark energy'? Or, what kind of mass (or matter) will ‘dark energy' turn into in the reversible situation? Does ‘dark energy' become ‘dark matter' (or dark mass in this case) - (probably no connection); or ordinary mass (or matter) - (highly unlikely); or some other kind of mass (or matter) - (maybe ‘doesn't matter')?

You'll note I'm actually using the terms of mass and matter interchangeably, although Einstein's famous equation has M = mass. However, IMHO, you can't have mass-less matter; all matter has mass, and if you gain too much mass (or matter) you go on a diet!

Here I will go off the deep speculative end, and suggest, for the sake of symmetry, that just as three known (and probably four) forms of matter produce positive (pulling) gravity, so too should four forms of matter be associated with negative (pushing) ‘gravity', or antigravity, now commonly called ‘dark energy'.

Beyond that statement I'm unable to wax lyrical – perhaps theoretical particle physicists (and kin) might be able to come up with exotic candidates. I mean particle physicists not only love symmetry, but are well into the theory of super-symmetry (SUSY) as being part and parcel of their quest to uncover a unification of all the known forces in nature. In addition to SUSY, throw in aspects of string or superstring theory, and/or M-Theory, and/or extra and hidden dimensions which may have a bearing on things, and who knows what sort of impossible things they (theoretical particle physicists) can come up with for us to believe in before breakfast!

So, four forms of matter (all symmetrical – two baryonic; two non-baryonic; two positive; two anti) produce positive gravity; four forms of other types of exotic matter (all symmetrical in like parallels) are associated with ‘dark energy'; and gravity and ‘dark energy' are also like the yin and yang. Symmetry rules, O.K.?

Of course, for all practical purposes, not all eight forms will actually exist – at least not in any significant amounts. There's little, for example, ordinary (baryonic) antimatter about (that's a mystery in itself) and probably little or no non-baryonic ‘anti dark matter'. Relative amounts aren't the point, rather the idea that Mother Nature has all these forms of matter potentially up Her sleeve. 

In addition to ‘dark energy' perhaps being suggestive of additional forms of matter, I strongly suspect that ‘dark energy' implies the existence of a fifth force in our Universe. Recall that the four known forces are 1) gravity; 2) electromagnetism; 3) the strong nuclear, and 4) the weak nuclear force. The strong and weak nuclear forces operate on much too short a distance scale (atomic lengths) for either to account for ‘dark energy'. ‘Dark energy' is unlikely to be electromagnetic in nature – otherwise it wouldn't be ‘dark' (photons anyone?). ‘Dark energy' could hardly be due to attractive gravity since ‘dark energy' is repulsive gravity or antigravity.  Now the particle that is assumed to convey the gravitational force is called a ‘graviton'. Perhaps there is such a thing as an anti-graviton particle, but then we haven't even trapped a graviton yet – it's still a beastie in the crypto-zoology realm of theoretical particle physics, so an anti-graviton is heaping speculation on another speculation and therefore probably not to useful at concept at this stage anyway. In any event, to date, we only know of anti-particles for the matter particles (electrons, protons, etc.) not the force carrying particles, so an anti-graviton is a real Twilight Zone concept in the here and now.

Anyway, if one assumes that none of the known forces can account for ‘dark energy', then the unification of physics, the Holy Grail ‘theory of everything' has gotten near infinitely more complicated!  When it comes to a ‘theory of everything', one needs to have all the jigsaw puzzle pieces present in the box before you can assemble it into a unified whole. Perhaps the ever constant failure of physicists to come up with a viable ‘theory of everything' is suggestive that we don't – to date- have all the jigsaw puzzle pieces. Anyway, all of this sounds like a good area for observational cosmologists and theoretical particle physicists to have a serious chinwag over several rounds of drinks to come to terms (if possible) with the elusive elements – extra forms of matter and/or of forces.

*Though I have suggested, rather light-heartedly, the replacement term ‘Upsidaisium'  after the ‘Rocky and Bullwinkle' cartoon serial which featured this antigravity metal as a more colorful term for ‘dark energy'. It's probably way too little too late however to find an alternative name for ‘dark energy' now.

About the Author:
Science librarian; retired.