  A recently investigated tachyonic scalar field dark energy dominated universe
exhibits a bivalent future: depending on initial parameters can run either into
a de Sitter exponential expansion or into a traversable future soft singularity
followed by a contraction phase. We also include in the model (i) a tiny amount
of radiation, (ii) baryonic matter ($\Omega _{b}h^{2}=0.022161$, where the
Hubble constant is fixed as $h=0.706$) and (iii) cold dark matter (CDM). Out of
a variety of six types of evolutions arising in a more subtle classification,
we identify two in which in the past the scalar field effectively degenerates
into a dust (its pressure drops to an insignificantly low negative value).
These are the evolutions of type IIb converging to de Sitter and type III
hitting the future soft singularity. We confront these background evolutions
with various cosmological tests, including the supernova type Ia Union 2.1
data, baryon acoustic oscillation distance ratios, Hubble parameter-redshift
relation and the cosmic microwave background (CMB) acoustic scale. We determine
a subset of the evolutions of both types which at 1$\sigma $ confidence level
are consistent with all of these cosmological tests. At perturbative level we
derive the CMB temperature power spectrum to find the best agreement with the
Planck data for $\Omega _{CDM}=0.22$. The fit is as good as for the $\Lambda
$CDM model at high multipoles, but the power remains slightly overestimated at
low multipoles, for both types of evolutions. The rest of the CDM is
effectively generated by the tachyonic field, which in this sense acts as a
combined dark energy and dark matter model.
