## 2.14 Outlook

Dark matter dominates the matter content of the universe, and only through astrophysical
and cosmological observations can the nature of dark matter on large scales be determined. In
this review, we have discussed a number of observational techniques available to Euclid: dark
matter mapping, complementarity with other astronomical observations (e.g., X-ray and CMB
experiments); cluster and galaxy scale dark matter halo mapping; and power spectrum analyses. The
techniques described will allow Euclid to constrain a variety of dark matter candidates and their
microphysical properties. We have discussed Warm Dark Matter scenarios, axion-like dark matter,
scalar field dark matter models (as well as the possible interactions between dark energy and
scattering with ordinary matter) and massive neutrinos (the only known component of dark
matter).
Here, we briefly list the main dark matter constraints so far forecasted for Euclid:

- The weak lensing power spectrum from Euclid will be able to constrain warm dark matter
particle mass to about [630];
- The galaxy power spectrum, with priors from Planck (primary CMB only), will yield an error
on the sum of neutrino masses of 0.04 eV (see Table 18; [211]);
- Euclid’s weak lensing should also yield an error on of 0.05 eV [507];
- [480] have shown that weak gravitational lensing from Euclid data will be able to determine
neutrino hierarchy (if );
- The forecasted errors on the effective number of neutrino species for Euclid (with a
Planck prior) are [for weak lensing 507] and [for galaxy clustering 211];
- The sound speed of unified dark energy-dark matter can be constrained with errors
by using 3D weak lensing [202];
- Recently, [633] showed that with current and next generation galaxy surveys alone it should be
possible to unambiguously detect a fraction of dark matter in axions of the order of 1% of the
total;

We envisage a number of future scenarios, all of which give Euclid an imperative to confirm or identify
the nature of dark matter. In the event that a dark matter candidate is discovered in direct
detection experiments or an accelerator (e.g. LHC) a primary goal for Euclid will be to confirm, or
refute, the existence of this particle on large scales. In the event that no discovery is made
directly, then astronomical observations will remain our only way to determine the nature of dark
matter.