What is Dark Matter?
Unlike normal matter such as stars and dust in our universe, dark matter can neither emit nor reflect light, and thus cannot be seen directly by telescopes. However we have several reasons to believe that dark matter exists in our universe. The evidence for non-visible mass called dark matter was first inferred in the 1930’s from observations of the orbital velocities of galaxies. The estimated mass of galaxies was much larger than one expected from the observed matter, implying that most of the matter must be dark. Recently the Planck satellite has measured the cosmic microwave background very accurately, telling us that our universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy (see Fig. 1). We do not know what the nature of dark matter is, nor where it comes from.
How are we doing?
There are many experiments around the world including KIMS (Korean Invisible Matter Search) and COSINE to look for dark matter. Although we cannot see dark matter directly, we can detect it by measuring nuclear recoil energy from dark matter interaction with nuclei in our scintillating crystal detector.
The KIMS experiment
The KIMS group has formed in 1999 to search for dark matter using CsI(Tl) crystals as detectors (Figure 2). In 1999 and 2008, the DAMA experiment using NaI(Tl) crystals announced that they observed a strong evidence of a dark matter signal. Many groups, including us, have searched for the DAMA signal using better detection sensitivity, but found no signal. The DAMA signal has been a puzzle for more than 10 years.
The COSINE experiment
It is very important to confirm or refute the DAMA signal by performing exactly the same detection technique as used for DAMA. For this purpose, we have formed the COSINE experiment which uses ultra-low background NaI(Tl) crystals grown from highly radioactive pure powders.
DAMA’s claimed signal is an annual (seasonal) modulation, which is consistent with the standard halo model due to the Earth relative motion around the Sun and the Galactic Center if the signal was indeed from dark matter interactions.
To test the DAMA’s signal with an apple-to-apple approach, the COSINE-100 experiment with an array of 8 crystals (total mass 106 kg) has been built and is currently running at Yangyang underground laboratory (Figure 3). After collecting of at least two years of data with COSINE-100, it may be possible to either see an indication of these modulation signals if DAMA was right, or carve out their signal parameter space if there is no signal.