**Abstract**

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.

**Keywords:** dark matter, WIMP, MACHO, MOND, rotational curve, ultra-diffuse galaxies, gravitational lensing, milky way

### **1. Introduction**

In 1933 Fritz Zwicky [1] indicated a problem related to the galaxy cluster Coma. Galaxy cluster studied by Zwicky appeared to contain some 400 times more matter than an ordinary, visible, i.e., luminous matter. The content of the luminous matter was estimated form the amount of light emitted by the cluster. However, there was no response for that finding. Only 40 years later in 1970s the problem was rediscovered and concerned almost all of the galaxies. Research of Vera Rubin discovered that the galaxies rotate in a way that cannot be explained by taking into account visible, luminous matter. Today we know that most of the matter in the Universe is dark. Various attempts to resolve the problem of the existence of a mysterious form of matter, dark matter, have been taken ever since. One such idea is to find a particle to possibly complete the standard model. The most important property of such particle would be that it is not a subject to electromagnetic force; hence the dark matter is invisible in all electromagnetic wavelengths. In order to detect such particle, sensitive detectors are built, but still final conclusion has not been made. Another attempt of explaining the problem of missing matter was based on the assumption of existence of astrophysical objects such as black hole or dim brown dwarfs. This idea has rather been discredited as the abundance and masses of such objects are too small comparing to the amount of the matter that is missing. On different grounds stands the idea of modifying gravity in low acceleration regime. Modified Newtonian dynamics (MOND) proposed by M. Milgrom in 1983 is a phenomenological approach attempting to provide explanation of rotation of

galaxies without invoking hidden matter at all. Yet such an approach seems to be in tension with recent findings of van Dokkum et al. about the ultra-diffuse galaxies. There appear to exist galaxies devoided of dark matter—then what about MOND predictions? This contribution is completed with the rotational curve of the Milky Way determined with 3 m in diameter radio telescope in the Astronomical Observatory of the Jagiellonian University. Obtained rotational curve is flat which indicates the presence of dark matter in the halo of our galaxy.

rapidly that reflects an obvious fact that the velocity of a test object (a "star") increases as the effective gravitational force is growing (at a given radius, only the mass enclosed within a sphere of that radius is relevant in terms of excreting gravitational force—Newton's Shell Theorem). Past a certain distance though (when increasing a distance from the massive center of galaxy does not enclose adequately bigger amounts of mass), the effective force of gravity should decline (as R2 will increase faster than the mass enclosed in a sphere of a radius being that distance from the center so the force of gravity will decline) which should result in

Vera Rubin and Kent Ford published their first rotational curve in paper [2]. They presented there the rotation of Andromeda based on spectroscopic survey of emission regions applying neutral hydrogen, Hα, and [NII] λ6583 emission lines. Further works, see, e.g., [3], revealed that most of the galaxies have rather flat rotational curves like the one in **Figure 1**. The fact that more distant stars have almost constant velocity attracted the attention of scientists. The circular velocities of the stars are due to gravity which plays the role of centripetal force. Combining

Newton's law of gravity with an expression for centripetal force yields the

*GM <sup>R</sup>*<sup>2</sup> <sup>¼</sup> *<sup>V</sup>*<sup>2</sup>

where *G* is universal gravitational constant, *M* is mass exerting a gravitational force, *V* denotes velocity of a (test) object orbiting mass *M*, and *R* is the distance

*<sup>M</sup>* <sup>¼</sup> *GV*<sup>2</sup>

Since *G* is constant and *V* appears to be constant as we can see in rotational curves (see **Figure 1**), it would mean that the mass of a galaxy increases linearly

As we know most of galaxies including the Milky Way have a bright massive

supermassive black hole in the middle. The farther away from the center, the fainter the regions are, i.e., less stars hence less matter is present, and linear dependence (3) is almost impossible to be obeyed. Computer simulations show that the galaxies move in a way we can observe them only if there is another than ordinary, luminous, form of matter, namely, dark matter. The amount of dark matter should be as large as almost five times more than the amount of ordinary matter. This is in agreement with calculations made within lambda-cold dark matter model (Λ-CDM) and the data from Wilkinson Microwave Anisotropy Probe (WMAP) [4] as well as Planck mission [5]. Λ-CDM model is a parametrization of the Big Bang cosmological model in which the Universe contains three major components: first, a cosmological constant denoted by lambda (Greek Λ) and associated with *dark energy*; second, the postulated *cold dark matter* (abbreviated CDM); and third, *ordinary matter*. It is often referred to as the standard model of Big Bang cosmology because it is the simplest model that provides a reasonably good description of the content of the Universe. WAMP was a satellite designed to map the cosmic microwave background (CMB) radiation over the entire sky in five frequency bands. The agreement between Λ-CDM model and the data from WAMP is good enough, which supports the validity of this model [4, 5]. The Λ-CDM model indicates that the matter the

center, a *bulge*, with majority of stars placed in that range and possibly a

*<sup>R</sup>* , (1)

*R:* (2)

*M R*ð Þ� *R:* (3)

lower orbital velocities.

*Dark Matter within the Milky Way*

*DOI: http://dx.doi.org/10.5772/intechopen.90267*

following relation:

**205**

between them. One obtains from Eq. (1)

with the distance from its center:
