In the era of precision cosmology, one of the biggest challenges of modern cosmology is the elucidation of the origin of the late time acceleration of the Universe. The most straightforward candidate of its origin, often referred to as dark energy, is a cosmological constant, but there are other various candidates that are still worth considering and should be distinguished by future observations. In this talk, I will consider two interesting possibilities as other candidates of dark energy: One is the modification of gravity theory. We construct the general framework of the scalar-tensor theories that has been developed so far and show that the resultant theories generally break the screening mechanism only inside astrophysical bodies, which leads to a novel probe of dark energy. Other is the remnant of quantum tunneling of the Universe. When we consider the open Universe created by bubble nucleation, the residual of the ancestor vacuum, that is the vacuum energy of the supercurvature mode, can be regarded as a possible candidate of dark energy. we discuss the detectability of this new idea by future observations.