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We present a new investigation of the intergalactic medium (IGM) nearreionization using dark gaps in the Lyman-$\beta$ (Ly$\beta$) forest. With itslower optical depth, Ly$\beta$ offers a potentially more sensitive probe to anyremaining neutral gas compared to commonly used Ly$\alpha$ line. We identifydark gaps in the Ly$\beta$ forest using spectra of 42 QSOs at $z_{\rm em}>5.5$,including new data from the XQR-30 VLT Large Programme. Approximately $40\%$ ofthese QSO spectra exhibit dark gaps longer than $10h^{-1}{\rm Mpc}$ at$z\simeq5.8$. By comparing the results to predictions from simulations, we findthat the data are broadly consistent both with models where fluctuations in theLy$\alpha$ forest are caused solely by ionizing ultraviolet background (UVB)fluctuations and with models that include large neutral hydrogen patches at$z<6$ due to a late end to reionization. Of particular interest is a very long($L=28h^{-1}{\rm Mpc}$) and dark ($\tau_{\rm eff} \gtrsim 6$) gap persistingdown to $z\simeq 5.5$ in the Ly$\beta$ forest of the $z_{\rm}=5.85$ QSO PSOJ025$-$11. This gap may support late reionization models with a volume-weightedaverage neutral hydrogen fraction of $ \langle x_{\rm HI}\rangle \gtrsim 5\%$by $z=5.6$. Finally, we infer constraints on $\langle x_{\rm HI}\rangle$ over$5.5 \lesssim z \lesssim 6.0$ based on the observed Ly$\beta$ dark gap lengthdistribution and a conservative relationship between gap length and neutralfraction derived from simulations. We find $\langle x_{\rm HI}\rangle \leq0.05$, 0.17, and 0.29 at $z\simeq 5.55$, 5.75, and 5.95, respectively. Theseconstraints are consistent with models where reionization ends significantlylater than $z = 6$.

Long Dark Gaps in the Ly$\beta$ Forest at $z<6$: Evidence of Ultra Late Reionization from XQR-30 Spectra