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Belief in a scientific theory must always be established on an objective assessment of the evidence. That several lines of evidence give the same numbers is not a perfect guarantee of correctness - Kelvin was sure he knew the age of the sun and solar system because his calculation of the cooling age of the earth gave the same 10-20 Myr as the lifetime of a solar mass star with gravitational contraction as its only energy source.

There are respectable motivations to take the Lambda-CDM model seriously as a hypothesis about the universe, but this is not equivalent to declaring its unvarnished truth.

4.1. Falsifiability

For many years, the Einstein-de Sitter model was the most popular hypothesis for a dynamical description of the universe. The high redshift Type Ia supernovae were a strong evidence supporting its inconsistency. Today the evidence against this once favoured hypothesis comes from many different observations.

But the important thing of the present standard model (Lambda-CDM) is that it can make predictions that can be tested by observations and therefore the theory is falsifiable (vulnerable to being shown false by observation or experiment). An example from the past in cosmology: steady state was falsified (fairly quickly, in fact, as we had already explained) because it made some definite predictions. An example for hopefully the near future: of the popular ideas out there now, inflation is surely falsifiable

a) via polarization structure of CMB and such, indeed it is looking a little weak in the knees now: polarization-sensitive CMB experiments will come very soon.

b) via detection of a stochastic gravitational wave background.

4.2. Consensus?

The consensus about the existence of dark matter is high. The evidence of its existence is clearly stronger than the evidence of the existence of dark energy. Prospects for the detection of dark matter candidates are ongoing in different experiments. There are interesting, well-motivated DM candidates (and also of course some silly ones), being the neutralino the everyone's favourite candidate for the moment (Kolb 2007b).

One of the first alternatives to dark matter was formulated by Finzi (1963) to guarantee the stability of clusters of galaxies without advocating for dark matter. Finzi's hypothesis was a modification of the gravitational Newton law in such a way that the actual attraction at long distances should be stronger than the value predicted by the Newton's Law, but probably the optimal version of this has not yet been put forward. Two decades later, Milgrom (1983) proposed a different alternative to the dark matter based in a modification of Newtonian dynamics (or MOND for short). In this hypothesis, the Newton's second law of dynamics is modified in such a way that when accelerations experienced by objects are smaller than a certain value, the gravity force is inversely proportional to the distance, instead of to the distance squared. This modification explains rather well the flat rotation curves of the spiral galaxies (Sanders & McGaugh 2002) whose dynamics are a consequence of the luminous baryonic matter alone, with no need to claim for dark matter. Although MOND has successfully explained other cosmological observations, it does not reproduce so well the dynamics of clusters of galaxies and the observations of weak and strong lensing and the CMB.

4.3. Success?

Should we regard the "discovery" of the dark energy and the acceleration of the expansion in the universe as a scientific success? Certainly in 1998, Science magazine considered this discovery as the breakthrough of the year and we agree with that decision. This discovery put together many astrophysicists, cosmologists and high-energy physicists in a common effort trying to understand the nature of the dark energy (see for example the Dark Energy Task Force report by Albrecht et al. (2006) and the ESA-ESO Working Group on "Fundamental Cosmology" by Peacock et al. (2006)). But is the discovery of the dark energy by itself a scientific success? It is certainly a crucial step, but probably the story should not be considered a success at least until it can be well explained in terms of an existing theory. As Lee Smolin (2006) says: "The discovery of the dark energy cannot be counted as success, for it suggests that there is a major fact that we are all missing." Of course, this statement does not subtract the merit to the Supernova Cosmology Project led by Saul Perlmutter and the High-z Supernova Search Team led by Brian Schmidt and other observations supporting the accelerating world models; what it means is that the presence of a non-zero vacuum energy is a problem that has to be explained in much the same way the existence of the aether was a problem that had to be explained by the physicists prior to the Michelson-Morley experiment. In that case the experiment acted denying the existence of the aether and that was the solution. In Cosmology, future planned and ongoing observations have as a major scope to understand the nature of the dark energy (Albrecht et al. 2009). Some of these projects are based on distant supernovae (Wood-Vasey et al. 2007) and BAOs (Benítez et al. 2009, Glazebrook et al. 2007, Schlegel et al. 2009, Cimatti et al. 2009).

In any case and if what you care about is things like galaxy formation and evolution, then DE was not important when most of the relevant processes were going on. As White (2007) has remarked in a recent essay DE is an interesting problem to plan astronomical observations, but it is just "one of many."

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