There are really few questions in science more interesting than that of finding out, which is the nature of the matter composing the Universe. It is amazing that after so much effort dedicated to such question, what is the Universe composed of? it has not been possible to give a conclusive answer. From the latest observations, we do know that about 96% of matter in the Universe is of non-baryonic nature. The old belief that matter in Cosmos is made of quarks, leptons and gauge bosons is being abandoned due to the recent observations and the inconsistencies which spring out of this assumption \cite{schram}. Now we are convinced on the existence of an exotic non-baryonic sort of matter which dominates the structure of the Universe, but its nature is until now a puzzle.

Recent observations of the luminosity-redshift relation of Ia Supernovae suggest that distant galaxies are moving slower than predicted by Hubble's law, that is, an accelerated expansion of the Universe seems to hold \cite{perlmutter,riess}. Furthermore, measurements of the Cosmic Background Radiation and the mass power spectrum also suggest that the Universe has the preferable value W0=1. There should exist a kind of missing anti-gravitational matter possessing a negative pressure p/r= w < 0 \cite{ostriker} which should overcome the enormous gravitational forces between galaxies. Moreover, the interaction with the rest of the matter should be very weak to pass unnoticed at the solar system level. These observations are without doubt among the most important discoveries of the end of the last century, they gave rise to the idea that the components of the Universe are matter and vacuum energy W0 = WM + WL. Models such as the quintessence (a slow varying scalar field) imply -1 < w< 0 and the one using a cosmological constant, requiring w = -1, appear to be strong candidates to be such missing energy, because both of them satisfy an equation of state concerning an accelerated behavior of the Universe \cite{stein}

Observations in galaxy clusters and dynamical measurements of the mass in galaxies indicate that WM ~ 0.3, (see for example \cite{turner). Observations of Ia supernovae indicate that WL ~ 0.7 \cite{perlmutter,riess}. These observations are in very good concordance with the preferred W0 ~1. Everything seems to agree. Nevertheless, the matter WM decomposes itself in baryons, neutrinos, etc. and dark matter. It is observed that stars and dust (visible baryons) represent something like 0.3% of the whole matter of the Universe. The new measurements of the neutrino mass indicate that neutrinos contribute with about the same quantity as luminous matter. In other words, WM = Wb + Wn + ... ~ 0.04 + WDM , where WDM represents the dark matter part of the matter contributions which has a value WDM ~ 0.23. This value of the amount of baryonic matter is in concordance with the limits imposed by nucleosynthesis (see for example \cite{schram}). But we do not know the nature neither of the dark matter WDM nor of the dark energy WL; we do not know what is the composition of WDM + WL ~ 0.96, i.e., the 96% of the whole matter in the Universe.

In a series of previous works we have shown that the scalar field is a strong candidate to be the dark matter in spiral galaxies \cite{siddh}. Using the hypothesis that the scalar field is the dark matter in galaxies, we were able to reproduce the rotation curves profile of stars going around spiral galaxies. In fact the scalar potential arising for the explanation of rotation curves of galaxies is exponential. Moreover, by using a Monte Carlo simulation, Hurterer and Turner have been able to reconstruct an exponential potential for quintessence, which brings the Universe into an accelerating epoch \cite{turner1}. In this last work there is no explanation for the nature of dark matter, it is taken the value WDM ~ 0.23 without further comments. Recently, there are other papers where the late time attractor solutions for the exponential potential are studied \cite{ferr,maco,barr}. If we are consistent with our previous work, this dark matter should be also of scalar nature representing the 35% of the matter of the Universe. We have shown that the hypothesis that the scalar field is the dark matter and the dark energy of the Universe is consistent with Ia supernovae observations and it could imply that the scalar field is the dominant matter in the Universe, determining its structure at a cosmological and at a galactic level. In other words, we demonstrate that the hypothesis that the scalar field represents more than 96% of the matter in the Universe is consistent with the recent observations on Ia supernovae, the mass power spectrum, the angular power spectrum, putting the scalar field dark matter model at the same level as the LCDM one. See also:

Class. Quant. Grav.17,(2000) L75-L81. Available at: astro-ph/0004332;

Phys Rev.D62,(2000),081302(R). Available at: astro-ph/0003364;

Phys Rev.D63,(2001),063506. Available at: astro-ph/0006024;

Phys Rev.D66,(2002),023514. Available at: astro-ph/0111292;

Phys. Lett.B538,(2002),246-250. Available at: astro-ph/0010226.

References
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{barr} T. Barreiro, E.J. Copeland and N.J. Nunes, astro-ph/9910214.