  The result of a search for pair production of the supersymmetric partner of
the Standard Model bottom quark ($\tilde{b}_1$) is reported. The search uses
3.2 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=$13 TeV collected by the ATLAS
experiment at the Large Hadron Collider in 2015. Bottom squarks are searched
for in events containing large missing transverse momentum and exactly two jets
identified as originating from $b$-quarks. No excess above the expected
Standard Model background yield is observed. Exclusion limits at 95% confidence
level on the mass of the bottom squark are derived in phenomenological
supersymmetric $R$-parity-conserving models in which the $\tilde{b}_1$ is the
lightest squark and is assumed to decay exclusively via $\tilde{b}_1
\rightarrow b \tilde{\chi}_1^0$, where $\tilde{\chi}_1^0$ is the lightest
neutralino. The limits significantly extend previous results; bottom squark
masses up to 800 (840) GeV are excluded for the $\tilde{\chi}_1^0$ mass below
360 (100) GeV whilst differences in mass above 100 GeV between the
$\tilde{b}_1$ and the $\tilde{\chi}_1^0$ are excluded up to a $\tilde{b}_1$
mass of 500 GeV.
