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The supermassive black hole located in the galaxy M87 (BH M87) is four times larger than our solar system. If it is spherically symmetric, then a capsule free falling from a distance of 1 light year would cross BH M87’s event horizon within some tens of years. Continuing that journey, any unfortunate astronomer traveling within the capsule would remain alive for a few further tens of hours; if the capsule were equipped with a powerful engine and could slow down, their lifetime inside the horizon beyond “the gates of Hell” would be slightly extended. How is this so? What are the other properties of the interior of BH M87? Maintaining the assumption of spherical symmetry of the exterior of BH M87, we briefly discuss some simple but intriguing properties of its interior, a region that turns out to be highly anisotropic, both expanding and contracting at the same time.

One can question the relationship between the symmetries of the exterior and interior of black holes with an isotropic and static exterior. This question is justified by the variety of recent findings indicating substantial or even dramatic differences in the properties of the exterior and interior of isotropic, static black holes. By invoking some of these findings related to a variety of the thought experiments with freely falling or uniformly accelerated test particles, one can establish the dynamic properties of the interior, which turn out to be equivalent to anisotropic cosmology, simultaneously expanding and contracting, albeit in different directions. In order to illustrate the comparison between the symmetry of the exterior vs. the interior, we apply conventional t, r, θ, φ coordinates to both of these ranges, although on the horizon(s) they display singular behavior. Using a simple approach based on co-moving and freely falling observers, the dynamics of the cylindrically shaped interior are explored. That enables us to present schematic snapshots of the interior of a Schwarzschild black hole, expanding along its cylindrical axis and contracting along its spherical base, as well as the interior of a Reissner−Nordström black hole, expanding first and then contracting along the cylindrical axis up to the terminal instant r =r−.

Large black holes of millions of solar masses are known to be present in the centre of galaxies. Their mass is negligible compared to the mass of the luminous matter, but their entropy far exceeds the entropy of the latter by 10 orders of magnitude. Strong gravitational fields make them ‘black’—but at the same time, they cause them to emit radiation—so they are not ‘dark’. What is the meaning of their borders that may only be crossed once and that leads to the information paradox and what are the properties of their interiors? In discussing these and related questions (is it possible that the volume of a black hole might be infinite?), we uncover the unexpected meaning of the term ‘strong gravity’.

Dark matter is an invisible substance that seems to make almost 85% of all the mass and roughly 26% of mass-energy content of our Universe. We briefly present the history of its discovery, and we discuss the main attempts to resolve the problem of the origin of dark matter. Those attempts are as follows: dark matter particles (WIMPs), unseen astrophysical objects (MACHOs), or interactions of dark matter with ordinary (luminous) matter. We also introduce a different approach claiming no need for existence of the dark matter (MOND) and recent findings about the ultra-diffuse galaxies. Finally we present 21-cm line observations of neutral hydrogen in the Milky Way made by using 3 m in diameter radio telescope in the Astronomical Observatory of the Jagiellonian University. These studies yield rotational curve of our galaxy. Rotational curve we obtained is compared to those present in literature and constitutes a proof of presence of dark matter in the Milky Way.