<p>A huge number of neutrinos is produced in nuclear reactions inside the Sun, and quickly escapes from our star in all directions. They are so many that every squared meter of the Earth&#39;s surface facing the Sun is crossed by about 3 million billion (yes, 3 10<span>15</span><span>) neutrinos&nbsp;</span><em>every second</em><span>! Which means that during our lifetime, our body receives as a whole something like 10</span><span>21</span><span>&nbsp;(a thousand billion billion) neutrinos. This is an unimaginable quantity, but thanks to the weakness of neutrino interactions with matter, we don&#39;t notice this at all! Indeed, most of the neutrinos just pass through our planet completely undisturbed, and it is estimated that only 1 or 2 of those worrying 10</span><span>21</span><span>&nbsp;happen to meet one of the particles we&#39;re made of.</span></p>

<p>This is another consequence of neutrinos&#39; elusivity: both photons and neutrinos are produced by the nuclear reactions taking place in the core of the Sun. Afterwards, they need to reach the surface of the star before being able to travel in the empty space towards us., which, as you probably know, takes about 8 minutes to light, and almost the same to neutrinos. It is exactly the journey from the nucleus to the surface that makes all the difference: for neutrinos, the Sun is transparent, so it is very fast for them. Photons, instead, participate to the electromagnetic interaction, and therefore they have a much higher probability to interact with the electrons and nuclei that compose the solar plasma. This means that they have to walk a very very tortuous path through continuous absorptions, scatterings and re-emissions, until they slowly cross, one after the other, all the outer layers of the Sun. This takes on average millions years! So, what would happen if the Sun switched off in this instant? The huge amount of photons that are already on their way to the Sun&#39;s surface will just go on in their journey, and sunlight would be granted for some other million years. However, the absence of neutrino flux would be noticed by all the neutrinos detectors we have on Earth in less than 10 minutes.&nbsp;</p> <p><img alt="" src="/sites/default/files/images/nu_vs_light_sun.gif" style="width: 391px; height: 343px;" /></p>

<p>One of the leading candidates for dark matter is a type of particle that can have weak and gravitational interactions, but does not have electromagnetic or strong interactions. Such a particle is called Weakly Interacting Massive Particle (WIMP). On average at most one WIMP interacts with the human body per minute, although billions of them can pass through the body every second.</p>

<p>Many observational evidences suggest that galaxies contain an invisible form of matter which does not interact electromagnetically. The total mass of a galaxy can be measured from the velocities of the stars that orbit the center of the galaxy. We can measure the mass of the luminous part of galaxies, and thus can estimate the remaining mass which is invisible and is called dark matter. Dark matter is estimated to be more than 5 times the ordinary matter in the universe.</p>