(From: Begeman, Broeils, &Sanders 1991; Sellwood&McGaugh 2005.)

Let me try a different approach: The biggest problem with the acceptance of dark energy and dark matter is first of all the word “dark”, but we are stuck with that - indeed they both do not emit light and are thus dark.

Second biggest problem is that they are confused! Dark matter and energy are presented as if they are brothers, as if presenting merely the start of a new line of cheap ad hoc fudge factors: Dark energy, dark matter, dark force, dark my ass, what comes tomorrow? It is important for the public acceptance of both, dark energy and dark matter, to point out that dark energy is not at all a fudge factor and not even anything new.

Similar has been pointed out before, namely that dark energy is much less weird than usual pressure in Einstein’s general relativity:

Wait a minute! The crazy point [about dark energy] is that something that pulls itself together expands, right? But that is not the contribution of dark energy. That part is just usual general relativity. It is nothing else but what has been accepted for almost a century by now: Pressure, although it pushes and wants to expand, contributes to gravity, which pulls and leads to contraction.

Let me try a totally different way today:

Take a usual pendulum, like a massive bob hanging from a string, and watch it swinging. Somebody writes down the equation that describes this pendulum: It has mass M and the length of the string L and also a term for the gravity of the earth for example. You can see how simple these formulas are at Wiki's Harmonic Oscillator. Now say that you see the swinging slowly damp down. No problem: There is a damping term D in the equation that describes that. Now somebody opens the window. The wind turbulences due to the window’s shape render the air flow around the pendulum somewhat periodic. You see the pendulum swinging more again – it accelerates. It accelerates just slightly and you yourself do not even feel the wind at all. Mysterious?

Of course not. And the equation that describes this is very easily modified, too: Just add an applied force term F(t) to it. These terms in the equation of the harmonic oscillator, the damping term D and the driving term F(t), are completely natural. Especially, if you have another pendulum, say a vibrating string or a mass hanging from a spring, you of course apply the same or a very similar formula.

If you enter your observations of the swinging into this formula, it will tell you whether there is a damping going on: namely when the damping D turns out to be non-zero. It will also tell you whether there is a driving force that feeds the oscillation: Driving term F(t) is non-zero.

You would be a fool to look at the data of a pendulum and start to reject physics just because the observations tell you that the driving term F(t) is non-zero. All it says is: You might not yet know what exactly is driving the system, the wind or a magnet or whatever, but something is driving it.

And precisely, exactly, nothing else but what I just described goes on with dark energy! Dark energy is plainly coming from the classical theory of relativity known for almost a century by now. You have the usual equation and then you look at a particular system to which the equation is applicable, now the universe, and put your observational data, here from the observed expansion of space, into the formula. There is a driving term Λ (lambda) in there. Lambda is not put in there by hand, but it should be in there, just like there should be a damping term D and a driving term F(t) in your equation if you want to describe an oscillator that you come across somewhere. It is nothing strange, weird, or new.

Here is Einstein’s equation G^uv + Λ g^uv = T^uv, with the pressure-energy density tensor T on the right hand side and gravity, the G^uv, on the left side. Observation tells us that the lambda (Λ) you see there is non-zero. This is called dark energy or, if it turns out to be constant over time, also cosmological constant.

Does dark energy exist? YES! Of course it exists, because it is right there in the formula. Does the driving term in the formula of the harmonic oscillator exist? YES! It is right there on the paper where I wrote it down, and it would in a sense even exist if it turned out to be zero - but guess what, it is bigger than zero.

You may ask about what the fundamental nature of dark energy is: Is it quantum, vacuum energy for example? There are many unknowns, but it is not valid to say that dark energy does not exist or has not been observed. Of course it is observed. It is the observed data that leave the dark energy term non-zero! And that is dark energy – the lambda term does have the units of energy – just like the pressure-energy density tensor T right next to it (g has no units).

Some do not "believe" in dark energy. It is the same as if I measure the damping term D of a pendulum to be non-zero and then they blabber about that "friction" they cannot possibly believe in because what on earth is "friction" supposed to be.

And since you now know that it is childish to make a big fuss about dark energy, you know that everybody who throws dark energy and dark matter into one pot is already highly suspect of being a silly know-nothing. Dark matter is actually something that can be called a hypothesis, a good one by the way. The non-zero driving term F in the formula describing an oscillator is not a hypothesis, and it is a solid fact once it is observed to be non-zero.

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