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The properties of a remnant black hole can be probed by analyzing the gravitational waves emitted during its ringdown phase. This signal provides a direct test of general relativity in the strong-field regime. In this study, we apply both a time-domain <inline-formula><mml:math><mml:mrow><mml:mi>F</mml:mi><mml:mtext>-statistic</mml:mtext></mml:mrow></mml:math></inline-formula> framework and full Bayesian time-domain sampling to the ringdown of GW231028_153006. We report decisive evidence for multimode content, specifically identifying both the first overtone (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>221</mml:mn></mml:mrow></mml:math></inline-formula>) and the higher order (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>210</mml:mn></mml:mrow></mml:math></inline-formula>). The detection of the 221 mode is statistically significant, achieving a Bayes factor of <inline-formula><mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mn>189.2</mml:mn></mml:mrow></mml:math></inline-formula> and an amplitude exclusion from zero at <inline-formula><mml:math><mml:mrow><mml:mo>&gt;</mml:mo><mml:mn>7</mml:mn><mml:mi>σ</mml:mi></mml:mrow></mml:math></inline-formula> credibility for an analysis beginning at <inline-formula><mml:math><mml:mrow><mml:mn>10</mml:mn><mml:mi>M</mml:mi></mml:mrow></mml:math></inline-formula> postpeak. These findings are rigorously validated against the numerical relativity injection SXS:BBH:1282, which confirms that such multimode features are physically expected for a remnant with the inferred high spin and mass ratio. The inclusion of the overtone mode allows for precise constraints on the remnant's properties, yielding a redshifted final mass of <inline-formula><mml:math><mml:mrow><mml:mn>246</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>2</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>22.4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>22.3</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math></inline-formula> and a final spin of <inline-formula><mml:math><mml:mrow><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>81</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.10</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.07</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math></inline-formula> (at <inline-formula><mml:math><mml:mrow><mml:mn>90</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math></inline-formula> credibility), consistent with full inspiral-merger-ringdown predictions. A test of the no-hair theorem, enabled by this robust multimode detection, shows consistency with general relativity.

We place the first constraints on the dark matter fraction contained within dark photon solitons using the absence of their predicted radio-frequency signatures, or radio silence, following mergers around supermassive black holes. In these dense environments, spiky dark matter density profiles can form that enhance the soliton merger rate. We present a novel estimate of this rate by incorporating both the steepened dark matter profile and the soliton velocity dispersion via the Jeans equation. For galaxies with an initial profile ρ <SUB>DM</SUB> ∝ r <SUP>-1</SUP>, we find the total merger rate across redshifts 0 ≤ z ≤ 4 to be Γ<SUP>TOTAL</SUP> <SUB>merg</SUB> ≲ 10<SUP>-7</SUP> f <SUP>2</SUP> <SUB>DM</SUB> Mpc<SUP>-3</SUP> day<SUP>-1</SUP>, where f <SUB>DM</SUB> is the solitonic fraction of dark matter. This enhanced rate leads to more major merger events in which the generated soliton has a mass exceeding a critical threshold, leading to its decay via the parametric resonance phenomenon that produces brief, narrowband, and energetic radio bursts detectable by fast radio burst surveys. Comparing our predictions with the non-observation of such events, we already obtain f <SUB>DM</SUB> ≲ 10<SUP>-1</SUP> from the first fast radio burst study. This constraint is strengthened to f <SUB>DM</SUB> ≲ 10<SUP>-2</SUP> from the Parkes HTRU survey, with CHIME projected to tighten this to f <SUB>DM</SUB> ≲ 10<SUP>-3</SUP>. For larger f <SUB>DM</SUB>, we instead constrain the effective coupling strength between the dark and visible sectors to lie outside 10<SUP>-18</SUP> GeV<SUP>-1</SUP> ≲ g ≲ 10<SUP>-8</SUP> GeV<SUP>-1</SUP> for dark photon masses in the range 10<SUP>-6</SUP> eV ≲ m ≲ 10<SUP>-4</SUP> eV. Our results establish astrophysical transients as powerful probes of dark sectors, opening a window onto the detectability of ultralight vector fields.