We focus on two dark matter candidates: sterile neutrinos and primordial black holes (PBH). We explore the effects of non-standard pre-Big Bang Nucleosynthesis (pre-BBN) cosmologies, such scalar-tensor and kination cosmologies, on the abundance of sterile neutrinos over a large range of masses. In particular, sterile neutrinos of keV-scale mass represent a viable warm dark matter candidate whose decay can generate the putative 3.5 keV X-ray signal observed in galaxy and galaxy clusters. eV-scale sterile neutrinos can be the source of various accelerator/beam neutrino oscillation anomalies. Two production mechanisms are considered here, a collisional non-resonant Dodelson-Widrow (DW) mechanism and a resonant Shi-Fuller (SF) conversion (which requires a large lepton asymmetry). The DW mechanism is a freeze-in process, and the final abundance of sterile neutrinos using this production method is inversely proportional to the Hubble expansion rate. We find that in one of the scalar tensor models we consider, the sterile neutrino parameters necessary to generate the tentative 3.5 keV signal would be within reach of the TRISTAN upgrade to the ongoing KATRIN experiment as well as the planned upgrades to the HUNTER experiment, however the contribution to the dark matter density would be very small. In another scalar tensor model, sterile neutrinos could both generate the X-ray signal and comprise much of dark matter. In our study of resonant production, we find that the parameter space in which coherent and adiabatic resonant production can occur shifts with changing pre-BBN cosmology. We find that for a broad range of parameters (mass, mixing angle, lepton asymmetry), resonance can occur in the LSND/MiniBooNE and DANS/NEOSS experiments' preferred regions for at least one of the non-standard cosmologies we consider. With respect to PBH as dark matter candidates, we derive a new type of cosmology-independent bound. We consider the heating of the surrounding interstellar medium gas by dynamical friction and from the formation of accretion disks around intermediate mass $10-10^{5} M_\odot$ PBH. By estimating the cooling rate and assuming thermal equilibrium, we derive a new constraint. Light PBH with mass $10^{15}-10^{17}$ g emit significant Hawking radiation and are constrained by the same cooling argument. We extend this analysis to PBH with extreme spin, which results in stronger bounds compared to non-spinning PBH.

The channeling of the ion recoiling after a collision with a WIMP changes the ionization signal in direct detection experiments, producing a larger signal than otherwise expected. We give estimates of the fraction of channeled recoiling ions in NaI (Tl), Si, Ge, CsI, and solid Xe, Ar and Ne crystals using analytic models produced since the 1960's and 70's to describe channeling and blocking effects. We find that the channeling fraction of recoiling lattice nuclei is smaller than that of ions that are injected into the crystal and that it is strongly temperature dependent. Channeling is a directional effect which depends on the velocity distribution of WIMPs in the dark halo of our Galaxy and could lead to a daily modulation of the signal. We compute upper bounds to the expected amplitude of daily modulation due to channeling using our estimates of the channeling fractions. After developing the general formalism, we examine the possibility of finding a daily modulation due to channeling in the data already collected by the DAMA experiment. We find that even the largest daily modulation amplitudes would not be observable for WIMPs in the standard halo in the 13 years of data taken by the DAMA collaboration. For these to be observable the DAMA total rate should be 1/40 of what it is or the total DAMA exposure should be 40 times larger. The daily modulation due to channeling will be difficult to measure in future experiments.

We study two topics in this thesis: the halo-independent (HI) analysis method for dark matter (DM)-electron scattering and the phenomenology of sterile neutrinos ($\nu_s$s) produced in the evaporation of early Universe black holesWe demonstrate how the HI analysis can be applied to sub-GeV DM scattering off electrons for noble gas targets such as Xe and semiconductor targets such as Ge and Si. In the HI analysis method, properties of the local DM halo velocity distribution are inferred from direct DM detection data, which allows the comparison of different data sets without making any assumption on the uncertain local dark halo characteristics. This method had previously been developed for and applied only to DM scattering off nuclei. We additionally show that in-medium effects could significantly affect HI analysis results for semiconductor targets Ge and Si and thus are essential for proper inference of local DM halo characteristics from direct DM detection data. For $\nu_s$s as DM candidates, we discuss in detail the characteristics and phenomenology of $\nu_s$s that minimally couple only to active neutrinos and are produced in the evaporation of early Universe primordial black holes (PBHs), a process we call ``PBH sterile neutrinogenesis". Contrary to previously studied $\nu_s$ production mechanisms, this novel mechanism does not depend on the active-sterile mixing. The resulting $\nu_s$s have a distinctive spectrum and are produced with larger energies than in typical scenarios. This characteristic enables $\nu_s$s to be warm DM in the unusual $0.3$ MeV to $0.3$ TeV mass range if PBHs do not matter-dominate the Universe before evaporating. When PBHs matter-dominate before evaporating, the possible coincidence of induced gravitational waves associated with PBH evaporation and astrophysical X-ray observations from $\nu_s$ decays would constitute a distinct signature of our scenario.

A vast number of independent astrophysical and cosmological observations suggest that the dominant form of matter in the Universe, known as dark matter, is neither luminous nor baryonic. Despite nearly half a decade of research, the non-gravitational nature of dark matter, if any, remains a mystery. Motivated primarily by preferred theoretical extensions of the Standard Model and a relatively simple production mechanism, the weakly interacting massive particle (WIMP) has long been considered to be among the most appealing dark matter particle candidates. This dissertation is comprised of largely independent works that focus on understanding and constraining various signals that could arise from WIMP dark matter. Specifically, Chapters 2 and 3 address the impact that non-standard astrophysics and particle physics could have on the observed scattering rate in direct dark matter detection experiments; Chapter 4 presents a halo-dependent and an halo-independent update on the viability of a dark matter interpretation of the CDMS-II-Si data; Chapter 5 generalizes the halo-independent analysis formalism such that the compatibility of multiple experiments can be assessed, and the preferred halo-independent parameter space can be identified, for global likelihoods comprised of at least one extended likelihood; Chapter 6 discusses the prospects for detecting gamma-rays from dark matter annihilating in local dark matter subahlos; Chapter 7 presents updated constraints on simplified dark matter models that are consistent with the Galactic Center excess; and Chapter 8 discusses the extent to which future direct detection experiments may be able to elucidate the high-energy dark matter theory from observations of low-energy nuclear recoils.

Many dark matter candidates, including asymmetric Weakly Interacting Massive Particles (WIMPs) and sterile neutrinos, are produced in the very early Universe, prior to Big Bang Nucleosynthesis (BBN). We show that the relic abundance of asymmetric WIMPs and sterile neutrinos can be very sensitive to the expansion rate of the Universe prior to BBN. In particular, we find that if the production of asymmetric WIMPs occurs during a non-standard cosmological phase, a larger WIMP annihilation cross section is required to produce the present dark matter density than if the WIMPs were produced during a standard, radiation dominated phase. Because of this, the present dark matter annihilation rate could be larger than that of symmetric dark matter produced in the standard cosmology. We also show that if the production of sterile neutrinos occurs during a non-standard cosmological phase, the relic number density of sterile neutrinos could be reduced with respect to the number expected in the standard cosmology, consequently relaxing current bounds on active-sterile neutrino mixing. Finally, we examine whether low reheating temperature cosmologies are allowed by current Cosmic Microwave Background measurements. We find the allowed range of reheating temperatures using monomial and binomial inflationary potentials, and a variety of reheating models. We show that an inflationary model with a φ^1 potential and canonical reheating allows the possibility that dark matter could be produced during the reheating epoch, instead of when the Universe is radiation dominated.