# On a Possible Origin of the Gamma-ray Excess around the Galactic Center
## 1 Introduction and Motivation  
The physical nature of dark matter (DM) remains one of the biggest puzzles in modern physics despite the extensive searches for DM with colliders, direct detectors and indirect observations. The latter comprises the searches of various CR particles (see, e.g., [1]) and emissions from DM annihilation or decay, particularly in gamma rays (e.g., [2] part V). Possible gamma-ray signals from DM may in turn have two different spectral types: wideband continuum and narrow lines from annihilation directly into photons. The latter would represent a very pristine signature of DM, since such spectral features are not expected from any other astrophysical processes. Searches for narrow lines in the Fermi-LAT data were not successful so far--only upper limits were derived; see, e.g., [3]. This justifies a motivation to develop new gamma-ray telescopes with better spectroscopic capabilities. One such project, GAMMA-400, is being developed in Russia now [4]. It is expected to have better sensitivity to narrow spectral lines around GC in the energy range of \(\sim\)(0.1-100) GeV in comparison with Fermi-LAT due to better energy resolution. This and other aspects of DM searches by GAMMA-400 are described in detail in [5].  
There is less ignorance when it comes to the searches for continuum signals from DM annihilation or decay: already a while ago, a significant excess of gamma rays around GC was firmly identified (e.g., [6]). It is difficult to explain such diffuse excess by phenomenological models of emission from CRs, described, e.g., in the monographs [7, 8] and implemented in sophisticated numerical codes such as GALPROP [9, 10].  
The excess is spherically symmetric, visible up to \(\sim\)10\({}^{\circ}\) from GC and peaks around 2 GeV. It can be well fitted by annihilating WIMPs with the mass of several decades ofGeV, the annihilation cross-section around the thermal value, and steep density profile (e.g., [11; 12]). However, alternative explanations through various astrophysical mechanisms indeed exist. Thus, the excess can be caused by the unusual population of \(\sim\)1000-millisecond pulsars in the Galactic bulge--e.g., [13]. The pulsar interpretation is probably considered to be the most plausible at present. Other astrophysical interpretations include the emission from molecular clouds, the base of Fermi bubbles, a CR burst in the past etc. Furthermore, in our opinion, the secondary emissions from millisecond pulsars, produced by inverse Compton and bremsstrahlung of \(e^{\pm}\) ejected by pulsars, also deserve an attention. Thus, the DM interpretation is challenged by both its own systematics (e.g., [PERSON] et al. [14]) and the existence of many competing opportunities.  
Almost all of the mentioned opportunities were challenged at some point by a non-trivial observational fact: abnormal excessive emission was identified in other regions of the galactic plane [15]. This may indicate that the excess source is concentrated around the galactic plane rather than around GC. This in turn supports the molecular cloud (MC) origin of the excess originally proposed in [16], since the clouds indeed reside along all the plane. This creates a motivation to investigate this interesting opportunity further, which comprises the subject of our work. The next section describes in detail our model of CR interaction with MCs, which may modify the MC emission spectra through a peculiar way leading to the imitation of some excessive emission at GeV energies.