The article "Eight Magnifiers Relay to Take a Picture of Dark Matter Halo" published by science and technology hall, China on September 3, 2020 introduced the situation of taking pictures of dark matter, and posted relevant photos. So what is the so-called dark matter halo in these photos made of? What are their physical meanings? These so-called dark matter should only be normal matter with low temperature and little visible light, which we can call interstellar matter.The existence of interstellar matter directly excludes the possibility that Hubble redshift is Doppler effect, and at the same time, the existence of interstellar matter will cause astronomical observers on earth to actually observe only the secondary light produced by interstellar matter, rather than the primary light produced by celestial bodies. In the process of producing secondary light, the frequency of interstellar matter will decrease slightly every time it is regenerated. Therefore, when interstellar matter is regarded as macroscopic isotropic and basically uniform, the frequency of starlight will decrease in direct proportion to the distance, which is called Hubble redshift. This is the mechanism and truth of Hubble redshift.Since Hubble discovered in 1929 that the redshift of celestial bodies is proportional to the distance between celestial bodies and the earth, it is believed that the redshift of celestial bodies is the Doppler effect caused by the retrogression of celestial bodies, and then the redshift of celestial bodies is directly converted into the retrogressive velocity of celestial bodies and Hubble's law is deduced, in which the coefficient is called Hubble constant. But up to now, the values of Hubble constant measured by different methods are not completely the same. The emergence of this phenomenon may indicate that there is a fundamental error in Hubble's law: the fact that the redshift of celestial bodies is proportional to the distance is not the Doppler effect caused by the retrogression of celestial bodies, but the result of interstellar matter. This paper puts forward some analysis and discussion on this, which can be used for reference by interested friends.