# What Is Dark Matter?

The ratio between energy density and pressure is just a plain number.

What is the concept of dark energy, in layman's terms? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Well, you see, cosmologists (like astronomers*) are simple folks. They basically model everything as so-called “perfect fluids”.

What is a perfect fluid? It is stuff that has density and pressure, but nothing else (nothing else of interest to the cosmologist anyway). In other words, it is not sticky. It has no viscosity, no internal stresses, and its pressure is the same in all directions. In short, insofar as the cosmologist is concerned, it is fully characterized by these two numbers: density and pressure.

Not only that but here is the thing: No matter what units of measurement we use, energy density and pressure are measured using the same physical units. Pressure is force divided by area. Energy density is energy divided by volume. But energy is force times length; volume is area times length. So force divided by area is the same as energy (force times length) divided by volume (= area times length).

Therefore, the ratio between energy density and pressure is just a plain number. It is not measured in meters, feet, seconds, pounds, whatever. It’s just a so-called dimensionless number, often denoted by the letter w.

And this is where the simple-mindedness of cosmologists comes in: All they care about is this w. Various kinds of stuff are all characterized by this w, that is to say, the ratio of pressure to energy density.

For instance, stuff that has no pressure is just called “dust”. It doesn’t really have to be, you know, dust. A galaxy of stars is dust (more or less) since those billions of stars behave, insofar as the cosmologist is concerned, as particles of dust, i.e., as a medium with negligible pressure.

So what values of w are permissible? As it turns out, anything between +1 and –1 works. Yes, it means negative pressure is acceptable, too. This may sound like an oddity, but negative pressure basically characterizes stuff that requires an investment of work (energy) to make it expand, as opposed to expanding on its own and doing work, like hot gas in the cylinder of an engine.

So then: Dust includes pretty much all the matter that we know, but also matter that we do not know, the so-called dark matter. Stuff with pressure greater than 0 can be ignored in the present-day universe, because the density of such stuff decreases very rapidly; so even if there was such stuff in substantial quantities in the past, it has been diluted to near nothingness by now. That leaves stuff with negative pressure. Very specifically, stuff with extreme negative pressure, for which w = –1. This stuff has a very peculiar property: Even as the universe expands, its density remains constant, because all the work (energy) invested into making it expand ends up, in the end, making more of this stuff.

So the logic we follows goes like this: First, we note that space in our universe seems Euclidean: the sum of the angles of even a very large triangle is 180 degrees. The jargon is that the universe is “spatially flat”. This requires that the total density of all the stuff in the universe be its so-called critical density. But visible matter amounts to less than 5% of this critical density. So we ask a simple question: How much more stuff and what kind of stuff is needed to give the critical density, and also match other observed parameters of the universe? It turns out that we need about 30% “dust”, and the remaining 70% has to have that extreme negative pressure, with w = –1.

The missing matter (the difference between the 30% vs. the less than 5% that we know about) is called dark matter. The remaining stuff with negative pressure is called dark energy.

I hope this explanation makes it clear that all this is backed by pretty solid, rigorous mathematics; yet at the same time, we have not the faintest clue as to what these “dark” thingies really are.

Oh, we have theories. Lots of theories. Dark matter may be “WIMPs” (weakly interacting massive particles). Or whatever. As to dark energy, it could be just an artifact of the so-called cosmological constant. Or the potential energy of a so-called scalar field. Or maybe just the energy of the quantum vacuum (though when we attempt to calculate that, we find that we are off by dozens of orders of magnitude, which is rather embarrassing.)

Some believe that we are clever enough to eventually figure this out. Others (myself included) believe that we need to find ways to, you know, actually detect these dark thingies before we can say anything definite about their nature. And then there is the option that perhaps these dark thingies do not even exist, they are mirages of sorts because we are doing the math using the wrong theory of gravity. (Disclaimer: I also worked on modified theories of gravitation.) Then again, recent observations (in particular, gravitational wave observations and the direct observation of the shadow of the supermassive black hole M87*) put severe constraints on acceptable modified gravity theories.

So for now, “dark matter” and “dark energy” are just words used to label our ignorance. But at least we know their “equation of state”, the ratio between their pressure and energy density, this number w.

*Astronomers often talk about hydrogen, helium, and “metals”. That is to say, everything that is not hydrogen and helium in the periodic table counts as a “metal”. As I said, simple-minded folks.

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