The cosmic star formation history is well established increasing at high redshifts, peaking around z = 2 and decreasing until the present day. However, (i) the physical origin of this shape is not well understood and (ii) the contribution of dust obscured star formation and the nature of dusty star-forming galaxies is still debated. In order to tackle these problems we have exploited ALMA calibrator observations to perform a novel (sub)mm survey, ALMACAL. This survey comprises more than 2000h of observing time in all ALMA bands and configurations, which is more than the sum of all ALMA Large Programs. (i) By conducting a blind search for intervening molecular absorption lines detectable in the spectra of radio-bright background ALMACAL quasars, we are able put new strong constraints on the evolution of the cosmic molecular gas mass density, Omega_H2. Our results are complementary to those of targeted CO emission line surveys such as ASPECS and COLDz, but not sensitive to cosmic variance effects. (ii) With the unique ALMACAL dataset we can also study the impact of dust on galaxy evolution. To this end, we search blindly for submillimetre-bright galaxies in the calibrator fields, constructing the first accurate continuum number counts at 440 GHz (680 micron, ALMA Band 8) and resolving ~90% of the extragalactic background light at this frequency. This study provides us with important insights into dust obscured star formation.